Ferrous sintered alloy and process for producing the same as well as ferrous-sintered-alloy member

ABSTRACT

A process for producing ferrous sintered alloy according to the present invention is characterized in that it is equipped with: a compaction step of pressure compacting a raw-material powder in which an Fe-system powder is mixed with a reinforcement powder, thereby turning the raw-material powder into a powder compact; and a sintering step of heating this powder compact in an oxidation preventive atmosphere, thereby sintering the powder compact; and said reinforcement powder is an Fe—Mn—Si—C powder comprising an Fe alloy or an Fe compound that includes: Mn in an amount of from 58 to 70%; Si in an amount making a compositional ratio of the Mn with respect to the Si (i.e., Mn/Si) that is from 3.3 to 4.6; and C in an amount of from 1.5 to 3%; when the entirety is taken as 100% by mass. This Fe—Mn—Si—C powder is procurable inexpensively relatively; besides, ferrous sintered alloys, which are obtained using that, are better in terms of various characteristics than are conventional ferrous sintered alloys. Therefore, it is possible to intend to turn Cu-free ferrous sintered alloys, which are good in terms of their own characteristics, into low-cost ones.

TECHNICAL FIELD

The present invention is one which relates to a ferrous sintered alloy that is good in terms of strength and dimensional stability, and that makes it feasible to be free from Cu, or to be free from Ni, at low cost; and to a process for producing the same; as well as to a ferrous-sintered-alloy member that comprises that ferrous sintered alloy.

BACKGROUND ART

In order to cut the manufacturing costs of structural members such as mechanical parts, it is possible to think of utilizing a ferrous-sintered-alloy member in which a powder compact, which is made by pressure compacting a raw-material powder whose major component is iron, is heated to sinter it. When using a ferrous-sintered-alloy member, it becomes feasible as well to obtain products (or sintered bodies) that approximate the final configurations, and so it is possible to intend to reduce the manufacturing costs and material costs of structural members, due to cutting the machining processes, upgrading the materials' yields, and the like. In order for these, the strength of ferrous-sintered-alloy member, and the dimensional stability thereof before and after the sintering come to be important.

From such a viewpoint, Fe—Cu—C-system ferrous sintered alloys, in which powder compacts that comprise raw-material powders with Fe—Cu—C compositions are sintered, have been used heavily in applications for structural members. This is because Cu is an effective element in upgrading the strength of ferrous sintered alloy, and in stabilizing the dimensional accuracy before and after the sintering. Therefore, in the case of ferrous sintered alloy, Cu has been believed to be its essential element virtually, contrary to common iron and steel materials.

-   Patent Literature No. 1: U.S. Pat. No. 6,346,133; -   Patent Literature No. 2: U.S. Pat. No. 6,364,927; -   Patent Literature No. 3: Japanese Patent Gazette No. 3,309,970; -   Patent Literature No. 4: Japanese Unexamined Patent Publication     (KOKAI) Gazette No. 58-210,147; -   Patent Literature No. 5: Published Japanese Translation of PCT     Application Gazette No. 10-510,007; -   Patent Literature No. 6: Japanese Unexamined Patent Publication     (KOKAI) Gazette No. 2005-336,608; -   Patent Literature No. 7: Japanese Unexamined Patent Publication     (KOKAI) Gazette No. 2005-336,609; -   Non-patent Literature No. 1: High Strength Si—Mn-Alloyed Sintered     Steels, P.M. Int., Vol. 17, No. 1 (1985); -   Non-patent Literature No. 2: “Effect of Sinter-Hardening on the     Properties of High Temperature Sintered PM Steels,” Advances in     Powder Metallurgy & Particulate Materials, MPIF, 2002, part 13, pp.     1-13; and -   Non-patent Literature No. 3: “New focus on chromium may sidestep     alloy cost increases,” MPR., September (2004), pp. 16-19

DISCLOSURE OF THE INVENTION Assignment to be Solved by the Invention

However, Cu powders are higher in the unit prices, and their usage amounts in ferrous sintered alloys are greater comparatively. Accordingly, they of themselves eventually come to result in raising the production costs of ferrous sintered alloys. Furthermore, although Cu is an element that becomes the cause of hot brittleness in iron and steel materials, it is an element that is difficult to remove by smelting, and the like. Consequently, ferrous sintered alloys that employ Cu are disliked because of the accidental mingling into scraps, and so forth, and so are poor in the recyclability. Therefore, employing ferrous sintered alloys that include Cu has not been necessarily the one that is preferable, from a standpoint of environmental countermeasures that should be projected in order to effectively utilize resources.

In addition to Cu, Ni is available as an element that has been utilized heavily in ferrous sintered alloys. Similarly to Cu, Ni is also an element that is effective in upgrading the strength of ferrous sintered alloy, and so on. However, Ni powders are also expensive, and hence raise the production costs of ferrous sintered alloys. Moreover, since Ni is an allergic element as well, there may also arise cases where the employment of the same is not preferable.

In aforementioned Patent Literature Nos. 1 and 2 and Non-patent Literature Nos. 1, there are disclosed ferrous sintered alloys for which upgrading the strength, and the like, has been intended by containing Mn or Si therein without employing any Cu. However, they are those which are absolutely no more than on laboratory level, and differ from the present invention that will be described later, even in terms of the compositions of Mn and Si, and in terms of the addition methods, and so forth.

In Patent Literature No. 3, there is disclosed an ultra-high-density compaction method for powder compact.

In Patent Literature Nos. 4 through 7, there is disclosed a ferrous sintered alloy in which a mixed powder of a pulverized powder of Si—Mn—Fe parent alloy and an iron powder is compression compacted and sintered. However, when the ferrous sintered alloy being disclosed in these patent literatures is compared with a ferrous sintered alloy according to the present invention that will be described later, the former differs from the latter in terms of the following: the compositional ratio of Mn to Si (i.e., Mn/Si); whether or not C is contained substantially regarding the composition of a reinforcement powder itself to be employed, and the like.

Moreover, Patent Literature No. 5 even discloses a ferrous sintered alloy in which Mo is contained in substitute for Ni. However, its strength is not necessarily sufficient, and hence it requires a heat treatment, such as hardening or tempering, for making it be of high strength furthermore. It is needless to say that such a heat treatment raises the production cost of the ferrous sintered alloy, because it requires a great deal of time and man-hour.

In contrast to these, in Non-patent Literature No. 2 or 3, there is disclosed such a description that a high-strength ferrous alloy (or sinter hardening steel) is obtainable even while heat treatments after the sintering step is being omitted. However, Non-patent Literature No. 2 differs from the present invention, and so does not disclose any ferrous sintered alloy in which Mn and Si are contained. In Non-patent Literature No. 3, there is disclosed a sinter hardening steel that contains Cr, Mn, Si, and Mo. However, unlike a ferrous sintered alloy according to the present invention that will be described later, it is not one which is produced using a reinforcement powder, such as Fe—Mn—Si—C powders.

The present invention is one which has been done in view of such circumstances; and it is an object to provide a production process that makes it possible to obtain ferrous sintered alloys at lower cost, ferrous sintered alloys which make it possible to secure mechanical characteristics, such as strength, and a dimensional stability before and after the sintering, even while inhibiting the employment of Cu or Ni, and to provide ferrous sintered alloys like those, as well as to provide ferrous-sintered-alloy members comprising those ferrous sintered alloys.

Means for Solving the Assignment

The present inventors studied earnestly to solve this assignment; as a result of their repeated trial and error, they found out anew that it is possible to obtain ferrous sintered alloys, which are good in terms of the mechanical characteristics, such as strength, and the dimensional stability, at lower cost by means of using reinforcement powders (e.g., Fe—Mn—Si—C powders) whose compositions differ from those of the conventional ones, and then arrived at completing the present invention.

<<Process for Producing Ferrous Sintered Alloy>>

(1) A ferrous sintered alloy according to the present invention is a process for producing ferrous sintered alloy being characterized in that it is equipped with:

a compaction step of pressure compacting a raw-material powder in which an Fe-system powder comprising at least either one of pure iron or an iron (Fe) alloy is mixed with a reinforcement powder containing an alloying element other than Fe, thereby turning the raw-material powder into a powder compact; and

a sintering step of heating the powder compact in an oxidation preventive atmosphere, thereby sintering the powder compact; and said reinforcement powder is an Fe—Mn—Si—C powder comprising an Fe alloy or an Fe compound that includes:

-   -   manganese (Mn) in an amount of from 58 to 70% by mass         (hereinafter being simply referred to as “%”);         -   silicon (Si) in an amount making a compositional ratio of             the Mn with respect to the Si (i.e., Mn/Si) that is from 3.3             to 4.6; and         -   carbon (C) in an amount of from 1.5 to 3%;         -   when the entirety is taken as 100%.

(2) In the process for producing ferrous sintered alloy according to the present invention, a reinforcement powder that constitutes a raw-material powder comprises an Fe alloy or Fe compound that includes not only Mn and Si but also C. Besides, a ferrous sintered alloy that is obtained using a reinforcement powder (e.g., an Fe—Mn—Si—C powder) in which the compositional ranges of Mn, Si and C fall in the specific ranges as aforementioned exhibits good characteristics, such as mechanical characteristics (strength, elongation, hardness, and the like) and dimensional stability, even without employing any Cu powder or Ni powder, and so forth.

Furthermore, raw materials for that Fe—Mn—Si—C powder are far much better in terms of pulverlizability (or collapsibility) than that of conventional Fe—Mn—Si powders, and the like. Accordingly, an Fe—Mn—Si—C powder that is homogenous and fine can be obtained with ease relatively. By means of employing an Fe—Mn—Si—C powder whose granularity is fine and uniform like this, it becomes feasible to enhance the dimensional stability and mechanical characteristics of ferrous sintered alloy much more. Besides, an Fe—Mn—Si—C powder with the aforementioned compositional ranges, or its raw materials are procurable inexpensively, because they are used heavily as deoxidizing agents (silicomanganese, for instance) that have been employed at the time of steel making.

Therefore, in accordance with the production process according to the present invention, it is possible to use an Fe—Mn—Si—C powder that is good in terms of procurability and being low price, or its raw materials, without ever employing any Cu powder that is expensive relatively. Besides, it is possible to employ its raw materials, and the like, as fine powders that are homogenous, with ease relatively, because they are good in terms of pulverlizability. Therefore, it is possible to intend a greater cost reduction at the stage of procuring or preparing a raw-material powder. Besides, the resulting ferrous sintered alloy is good not only in terms of mechanical characteristics, and so forth, but also in terms of dimensional stability. Therefore, it is possible to intend not only to reduce the heat-treating costs for members that comprise the ferrous sintered alloy, but also to reduce the costs for processing them, and the like.

Hence, in accordance with the production process according to the present invention, it becomes feasible to reduce the production costs of ferrous sintered alloy or ferrous-sintered-alloy member remarkably all through the entire production step that begins with a raw-material step and arrives at a final product stage.

Furthermore, the ferrous sintered alloy being obtained by means of the present invention is one which surpasses conventional ferrous sintered alloys regarding the mechanical characteristics, and the like. Accordingly, when the required specifications for ferrous-sintered-alloy member are at comparable levels with conventional levels, the following become feasible as well: reducing a reinforcement powder in the employed amount per se; substituting an Fe-system powder for a much more inexpensive powder in which the amounts of alloying elements are less; and so forth. If such is the case, it becomes feasible to furthermore advance the reduction of production costs for ferrous sintered alloy or member comprising the same.

(3) By the way, in the case where the aforementioned reinforcement powder (e.g., an Fe—Mn—Si—C powder) is used, the following reasons or mechanisms, and the like, are not necessarily clear: why that powder is, or the raw materials are good in terms of pulverizability; or why the respective characteristics of ferrous sintered alloys that are obtained using that powder are able to upgrade more than those of conventional ones. According to studies done by the present inventors earnestly, they are believed as described below.

First of all, in addition to the compositions of Mn and Si (involving the ratio, Mn/Si), a feature, namely, an Fe—Mn—Si—C powder that is directed to the present invention contains C in a greater amount relatively, is believed to be a reason why it is more likely to be pulverized than conventional Fe—Mn—Si powders are. Specifically, it is believed to be so because, in addition to the intermetallic compounds of manganese and silicon (e.g., MnSi₃, and Mn₅Si₃), the carbides of manganese (e.g., Mn₂₃C₆, Mn₇C₃, and the like) also exist.

Next, reasoning why the ferrous sintered alloy that is obtained using an Fe—Mn—Si—C powder is good in terms of mechanical characteristics or dimensional stability, and the like, is believed to be as follows.

First of all, along with phosphorous (P) and sulfur (S), Mn, Si, and C that are included in an Fe—Mn—Si—C powder are called the five elements of steel, and are common reinforcement elements in iron and steel materials to be die cast or melt produced.

However, Mn and Si have been hardly employed substantially so far in the field of ferrous sintered alloy. Since Mn and Si are likely to make oxides whose affinity to oxygen is extremely high, it has been believed generally that they make ferrous sintered alloys in which the oxides have intervened inside the metallic structure. Such a circumstance is prominent in a case where Mn and Si are added in a form of powders, which are distinct from an Fe-system powder, into a raw-material powder. Although it is possible to think of using an Fe-system powder in which Mn and Si have been alloyed in advance, the resulting Fe-system powder has become very hard in that case so that the compaction of powder compact per se becomes difficult.

Hence, in the production process according to the present invention, Mn and Si are mixed in a raw-material powder to be present as reinforcement powders that are distinct from an Fe-system powder. And, the sintering of a powder compact that includes Mn and Si is carried out in such an oxidation preventive atmosphere that can sufficiently restrain Mn and Si from being oxidized (i.e., a sintering step).

In any event, the present inventors succeeded in obtaining a ferrous sintered alloy that exceeds conventional Fe—Cu (—C)-system ferrous sintered alloys, and which demonstrates mechanical characteristics that are at an equivalent level to those of carbon steel for machine structure, with use of an Fe—Mn—Si—C powder as a reinforcement powder, but without ever employing Cu or Ni.

Note that a compositional ratio of Mn to Si (i.e., Mn/Si) is limited as described above because of the following: intending to upgrade strengths with additions as less as possible; and making dimensional changes (or expansion magnitude) smaller.

<<Ferrous Sintered Alloy and Ferrous-Sintered-Alloy Member>>

The present invention can be grasped not only as the above-described production process, but also as a ferrous sintered alloy being obtained by means of that production process and a variety of members comprising that ferrous sintered alloy (or ferrous-sintered-alloy members).

(1) It is suitable that this ferrous sintered alloy (hereinafter, the “ferrous-sintered-alloy members” being involved) can comprise:

Mn in an amount of from 0.1 to 2.1%;

Si in an amount of from 0.05 to 0.6%

C in an amount of from 0.1 to 0.9%; and

the balance being made of Fe, and inevitable impurities and/or a modifying element;

when the entirety of that alloy is taken as 100%, for instance.

(2) Moreover, it is preferable that the ferrous sintered alloy can include an alloying element that upgrades its mechanical characteristics, and the like. As for such an alloying element, toughness, ductility, and so on, in higher dimension.

When naming one of such examples, it is suitable that the ferrous sintered alloy can comprise:

Mn in an amount of from 0.1 to 1.4%;

Si in an amount of from 0.05 to 0.4%

C in an amount of from 0.1 to 0.9%;

Cr in an amount of from 0.1 to 5%, and/or Mo in an amount of from 0.1 to 2%; and

the balance being made of Fe, and inevitable impurities and/or a modifying element;

when the entirety of that alloy is taken as 100%.

Here, Mn is an effective element in upgrading the strength of ferrous sintered alloy especially. When Mn is too less, its advantage is effected poorly. In reality, however, a ferrous sintered alloy with sufficient strength is obtainable even if Mn is a trace amount depending on the types of alloying elements being included in the raw-material powder. On the other hand, when Mn becomes excessive, the elongation of the resulting ferrous sintered alloy decreases so that the ductility declines, and then dimensional changes increase as well so that the dimensional stability is harmed. Hence, when the entire ferrous sintered alloy is taken as 100%, although the upper and lower limits of Mn are selectable arbitrarily within the aforementioned numerical range, it is preferable especially that numerical values, which are selected arbitrarily from the group consisting of 0.1%, 0.3%, 1.2%, 1.5%, 1.8% and 2.1%, can make the upper and lower limits.

Although Si contributes to upgrading the strength of ferrous sintered alloy, it greatly contributes to the dimensional stability of ferrous sintered alloy especially. In particular, this tendency appears greatly in a case where Si and Mn coexist with each other. Whereas Mn has such an action that tends to increase the dimensions of ferrous sintered alloy, Si has such an action that tends to decrease the dimensions of ferrous sintered alloy. It is believed that both of the elements that coexist with each other result in canceling those tendencies one another and then in securing the dimensional stability of ferrous sintered alloy. Si being too less is not preferable because the dimensional stability is effected poorly; Si being excessive is not preferable because the magnitude of dimensional shrinkage has enlarged. Hence, when the entire ferrous sintered alloy is taken as 100%, although the upper and lower limits of Si are selectable arbitrarily within the aforementioned numerical range, it is preferable especially that numerical values, which are selected arbitrarily from the group consisting of 0.05%, 0.1%, 0.4%, 0.5% and 0.6%, can make the upper and lower limits.

C is one of the important reinforcement elements in ferrous sintered alloy. It is needless to say that C diffuses during sintering so that ferrous sintered alloy undergoes solid solution strengthening; and additionally including C in a proper amount makes such heat treatments as the hardening and tempering for ferrous sintered alloy feasible; thereby making it possible to upgrade the mechanical characteristics of ferrous sintered alloy much more. When C is too little, its advantage is effected poorly; when C becomes excessive, the resultant ductility declines. Hence, when the entire ferrous sintered alloy is taken as 100%, although the upper and lower limits of C are selectable arbitrarily within the aforementioned numerical range, it is preferable especially that numerical values, which are selected arbitrarily from the group consisting of 0.1%, 0.2%, 0.3%, 0.8% and 0.9%, can make the upper and lower limits.

Furthermore, in the case of the present invention, it is possible to intend to turn high-strength ferrous sintered alloys into those having higher strengths with a lesser amount of C, compared with that in common carbon steel. Although the reason for this is not necessarily clear, it is deemed that Mn and Si affect it strongly. Concretely speaking, it is believed that the yield ratio of C is upgraded by manes of adding Mn and Si, and that the hardenability is also upgraded furthermore. In any event, it becomes feasible to secure high toughness while intending to turn ferrous sintered alloys into those with higher strengths, because it is possible to intend to turn them into those with higher strengths on a lower carbon-amount side than ever before. In other words, a ferrous sintered alloy is obtainable, ferrous sintered alloy in which the strength and toughness that are said to be in a trade-off relationship in general are made compatible with each other.

(3) The “modifying element” being referred to in the present description is an element other than Fe, Mn, Si and C (in addition those, Cr and/or Mo), and is an element that is effective in improving the characteristics of ferrous sintered alloy. Although the types of characteristics to be improved do not matter at all, the following are available: strength, toughness, ductility, dimensional stability, machinability, and the like. As a specific example of the modifying element, from 0.1 to 0.3% by mass of V, and so forth, are available. Moreover, it is even advisable to use a modifying compound, such as MnS, for the introduction of modifying element. In this case, it is preferable to set an amount of MnS from 0.1 to 0.5% by mass, for instance.

The respective elements can be combined with each other arbitrarily. The contents of these modifying elements are not limited to the exemplified ranges. Moreover, their contents are usually a trace amount, respectively.

The “inevitable impurities” are impurities that are included in a raw-material powder, and are impurities, and the like, which are mixed accidentally during the respective steps; and are elements that are difficult to remove in view of costs, or due to technical reasons, and so forth. In the case of being a ferrous sintered alloy that is directed to the present invention, P, S, Al, Mg, Ca, and so on, are available therefor, for instance. Note that, as a matter of course, the compositions of modifying element and inevitable impurities are not limited in particular.

(4) In the “ferrous sintered alloy” or “ferrous-sintered-alloy member” being referred to in the present description, its form does not matter at all. In particular, it is even allowable that the ferrous sintered alloy can be a workpiece that has a bulk shape, a rod shape, a tube shape or a plate shape, and the like, for instance, or it is also permissible that it can have a final configuration or can even be a structural member per se that can approximate it. In reality, however, since sintered materials have been usually used in order to aim at reducing the processing costs, and so forth, the configuration of ferrous sintered alloy (or member) can be approximated to a final-product configuration by means of (near) net shaping.

(5) Although the types of alloying elements that are included in the ferrous sintered alloy do not matter at all especially, not including Cu or Ni is more preferable than including them. This is because Cu-free ferrous sintered alloys that do not include any Cu substantially, and Ni-free sintered alloys that do not include any Ni are desirable in upgrading recyclability.

However, the present invention is not one which excludes cases where Cu and Ni are contained in the ferrous sintered alloy. The cases of containing Cu or Ni in adequate amounts along with the above-described Mn and Si are also involved in the range of the present invention.

The “mechanical characteristics” and “dimensional stability” being referred to in the present description depend on the composition of a raw material, the compacting pressure, the sintering conditions (e.g., temperature, time, atmosphere, and the like), and so forth. Therefore, it is not always possible to specify those “mechanical characteristics” and “dimensional stability” unconditionally. If they are referred to daringly, it is preferable that a tensile strength, one of the mechanical characteristics, can be 550 MPa or more, 600 MPa or more, and further 650 MPa or more, for ferrous-sintered-alloy members for general-purpose applications; and can be 850 MPa or more, 900 MPa or more, 950 MPa or more, and further 1,000 MPa or more, for ferrous-sintered-alloy members that are of high strength. Moreover, it is preferable that the dimensional stability can fall within ±:0.5%, within ±0.3%, within ±0.1%, and further within ±0.05%, by a rate of dimensional change before and after sintering. In addition, when it is referred to in relation to elongation, it is preferable that the elongation can be 0.5% or more, 1% or more, 1.5% or more, 2% or more, and further 3% or more.

(6) Unless otherwise specified, the designations, namely, “from ‘x’ to ‘y’” being referred to in the present description, involve the lower limit, “x,” and the upper limit, “y.” Moreover, the upper limits and lower limits being set forth in the present description are combinable arbitrarily, and are thereby able to constitute such a range as “from ‘a’ to ‘b’.”

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram that illustrates a configuration of a later-described tensile test specimen;

FIG. 2 is a graph that shows relationships between reinforcement-powder amounts and dimensional changes regarding ferrous sintered alloys that are directed to later-described Testing Example No. 1;

FIG. 3 is a graph that shows relationships between reinforcement-powder amounts and hardnesses regarding ferrous sintered alloys that are directed to Testing Example No. 1;

FIG. 4 is a graph that shows relationships between reinforcement-powder amounts and tensile strengths regarding ferrous sintered alloys that are directed to Testing Example No. 1;

FIG. 5 is a graph that shows relationships between reinforcement-powder amounts and elongations regarding ferrous sintered alloys that are directed to Testing Example No. 1;

FIG. 6 is a graph that shows relationships between FeMSIV-powder amounts and dimensional changes regarding ferrous sintered alloys that are directed to later-described Testing Example No. 2;

FIG. 7 is a graph that shows relationships between FeMSIV-powder amounts and hardnesses regarding ferrous sintered alloys that are directed to Testing Example No. 2;

FIG. 8 is a graph that shows relationships between FeMSIV-powder amounts and tensile strengths regarding ferrous sintered alloys that are directed to Testing Example No. 2;

FIG. 9 is a graph that shows relationships between FeMSIV-powder amounts and elongations regarding ferrous sintered alloys that are directed to Testing Example No. 2;

FIG. 10 is a graph that shows relationships between FeMSIV-powder amounts and dimensional changes regarding ferrous sintered alloys that are directed to later-described Testing Example No. 3;

FIG. 11 is a graph that shows relationships between FeMSIV-powder amounts and hardnesses regarding ferrous sintered alloys that are directed to Testing Example No. 3;

FIG. 12 is a graph that shows relationships between FeMSIV-powder amounts and tensile strengths regarding ferrous sintered alloys that are directed to Testing Example No. 3;

FIG. 13 is a graph that shows relationships between FeMSIV-powder amounts and elongations regarding ferrous sintered alloys that are directed to Testing Example No. 3;

FIG. 14 is a graph that shows relationships between sintering temperatures and tensile strengths regarding ferrous sintered alloys that are directed to later-described Testing Example No. 4; and

FIG. 15 is a graph that shows relationships between sintering temperatures and elongations regarding ferrous sintered alloys that are directed to Testing Example No. 4.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be explained in more detail while giving some of embodiment modes for the invention. Note that, involving embodiment modes below, contents being explained in the present description can be appropriately applied not only to the process for producing ferrous sintered alloy according to the present invention, but also to that ferrous sintered alloy (involving ferrous-sintered-alloy members). Specifically, in addition to the above-described constitutions, one or two or more constitutions, which are selected arbitrarily from the constitutions that are listed below, are capable of being further added to the production process according to the present invention, and to a ferrous sintered alloy being obtained by means of the same. The constitutions, which are selected from those set forth below, are addable to either one of the inventions, and are further addable thereto beyond the categories in a superimposed manner or arbitrarily. For example, it is needless to say that, when being one of the constitutions that relate to a composition of the ferrous sintered alloy, and the like, it can be relevant to the production process for the same. Moreover, although it appears at first glance to be a constitution that relates to a “production process,” it can be turned into a constitution that relates to a “ferrous sintered alloy” as well when comprehending it as a product-by-process. Note that whether or not any one of the embodiment modes is considered best depends on subject matters, their required performance, and so forth.

<<Raw-Material Powder>>

A raw-material powder comprises an Fe-system powder, one of the major components of the ferrous sintered alloy, and a reinforcement powder (e.g., an Fe—Mn—Si—C powder) that includes Mn, Si and C. Note that the Fe—Mn—Si—C powder will be referred to as an “FeMS powder” in what follows.

(1) Fe-System Powder

It is allowable that the Fe-system powder can be either a pure iron powder or an iron alloy powder, or can even be a mixture powder of them. It does not matter at all what alloying elements are included in the iron alloy powder. As these alloying elements, first of all, C, Mn, Si, P, S, and the like, are available. Although Mn, Si and C are added as the reinforcement element as well, it is permissible that they can be included in the Fe-system powder in a small amount, respectively. However, when the contents of C, Mn, Si and so forth increase, the resulting Fe-system powder becomes so hard that the compactibility declines. Hence, in a case where the Fe-system powder is an iron alloy powder, it is allowable to set up as follows: C in an amount of 0.02% by mass or less; Mn in an amount of 0.2% by mass or less; and Si in amount of 0.1% by mass or less.

As the other alloying elements to be included in the Fe-system powder, Mo, Cr, Ni, V, Co, Nb, W, and the like, are available. Since these alloying elements upgrade the heat treatabilities of ferrous sintered alloy, they are effective elements that strengthen the ferrous sintered alloy.

In particular, it is suitable that the raw-material powder can be prepared so that Mo makes from 0.1 to 2% by mass and/or Cr makes from 0.1 to 5% when the entire raw-material powder is taken as 100% by mass (hereinafter, being simply represented as “%” whenever appropriate). Although the upper and lower limits of Cr are selectable arbitrarily within its numerical range, it is preferable especially that numerical values, which are selected arbitrarily from the group consisting of 0.1%, 0.3%, 0.5%, 3%, 3.2% and 3.5%, can make the upper and lower limits. Moreover, although the upper and lower limits of Mo are selectable arbitrarily within its numerical range, it is preferable especially that numerical values, which are selected arbitrarily from the group consisting of 0.1%, 0.5%, 0.6%, 0.8%, 1%, 1.5% and 2%, can make the upper and lower limits. Note that these alloying elements can even be supplied as reinforcement powders that are distinct from the Fe-system powder, though it is preferable that the Fe-system powder can be good in terms of the handlability and homogeneity when they are included therein.

(2) FeMS powder

An FeMS powder that is directed to the present invention comprises an Fe alloy or an Fe compound that includes Mn in an amount of from 58 to 70%, Si in an amount that makes an Mn/Si ratio being from 3.3 to 4.6, C in an amount of from 1.5 to 3%, and the balance being Fe principally, when the entire FeMS powder is taken as 100%. By using this FeMS powder, it is possible to produce the ferrous sintered alloy, which is good in terms of the mechanical characteristics and dimensional stability, at lower cost.

When the amounts of Mn, Si and C are too small, the resulting raw material for the FeMS powder (or FeMS raw material) has turned into an alloy with ductility, and so it becomes difficult to pulverize it into a fine powder. Moreover, an addition amount of the FeMS powder becomes much in the raw-material powder, and hence the cost of the ferrous sintered alloy has risen. On the other hand, an FeMS powder, or a raw material, with too much Mn, Si or C is not preferable, because the procurement cost has risen. For reference, when making a remark regarding the pulverizability of that FeMS powder, the presence of C is important especially.

Hence, although the upper and lower limits of Mn in the FeMS powder are selectable arbitrarily within the aforementioned numerical range, it is preferable especially that numerical values, which are selected arbitrarily from the group consisting of 58%, 60%, 65%, 68% and 70%, can make the upper and lower limits when the entire FeMS powder is taken as 100%. Moreover, although the upper and lower limits of C in the FeMS powder are selectable arbitrarily within the aforementioned numerical range, it is preferable especially that numerical values, which are selected arbitrarily from the group consisting of 1.5%, 2%, 2.5% and 3%, can make the upper and lower limits. And, although the upper and lower limits of Mn/Si in the FeMS powder are selectable arbitrarily within the aforementioned numerical range, it is preferable especially that numerical values, which are selected arbitrarily from the group consisting of 3.3, 3.6, 3.8, 4.2, 4.4 and 4.6, can make the upper and lower limits.

In the FeMS powder, it is preferable that an amount of 0 being contained can be 1.5% or less, 1.2% or less, 1% or less, and further 0.8% or less. When the amount of 0 in the raw-material powder increases, the reinforcement action resulting from Mn and Si cannot be demonstrated sufficiently. Furthermore, in a case where a powder compact with such an ultra high density that a green-compact density ratio (or ρ/ρ₀), namely, a ratio of an apparent density (ρ) of the powder compact with respect to a theoretical density (ρ₀) thereof exceeds 96%, O that exists in its interior is not preferable, because it becomes a cause of giving rise to swells (or blisters) in the resulting sintered body.

(3) Although a proportion of the FeMS powder to be blended in the raw material powder differs in compliance with a composition of the FeMS powder or a desirable characteristic of the ferrous sintered alloy (or a composition of the ferrous sintered alloy), it is allowable to be blended in an amount of from 0.05 to 3% when the entire raw-material powder is taken as 100% by mass.

When a blended amount of the FeMS powder is too small, it is not possible to improve the characteristics of the resulting ferrous sintered alloy; whereas being too much is not preferable because the dimensional stability or elongation of the resultant ferrous sintered alloy might possibly decline. Although the upper and lower limits of the blended amount of the FeMS powder are selectable arbitrarily within that numerical range, it is preferable especially that numerical values, which are selected arbitrarily from the group consisting of 0.05%, 0.1%, 0.2%, 0.3%, 2.1%, 2.5% and 3%, can make the upper and lower limits when the entire raw-material powder is taken as 100% by mass.

(4) The smaller a particle diameter of the FeMS powder is, not only the more the green-compact density ratio and a sintered-body density ratio (or ρ′/ρ₀′), namely, a ratio of an apparent density (ρ′) of a sintered body with respect to a theoretical density (ρ₀′) thereof, upgrades, but also the more the dimensional stability and various mechanical characteristics, and the like, tend to upgrade as well. This reason seems to result from the following: it is likely to obtain homogenous ferrous sintered alloys in which compositional variations or segregations occur less; however, that reason is not necessarily settled at present.

By the way, it is difficult in general to produce a powder with a smaller particle diameter, or it is produced at higher cost. However, the FeMS powder that is directed to the present invention can be produced at lower cost, because it is likely to turn into a fine powder with ease relatively. Even when FeMS raw materials are used as being pulverized, this FeMS powder can turn into a powder whose particle diameter is 45 μm or less (or −45 μm) approximately, for instance. This is a sufficiently small particle diameter, even compared with those of conventional reinforcement powders.

In reality, however, it is preferable rather to inhibit the particle diameter from fluctuating and then use a fine powder with a much smaller granularity in upgrading the characteristics of the ferrous sintered alloy, or in stabilizing the qualities. Hence, it is suitable to use an FeMS powder that has been classified by means of sieving, and the like. Concretely speaking, it is suitable to use an FeMS powder that is classified to 30 μm or less, 20 μm or less, 10 μm or less, 8 μm or less, and further to 6 μm or less, and so forth, in addition to being classified to 45 μm or less, for instance. Although not especially persisting in the lower limit of the particle diameter of an FeMS powder, it is preferable that it can be 1 μm or more, and further 3 μm or more, in view of the handlability and production cost.

Note that, as an index for evaluating the size of particles in an FeMS powder, it is possible to use an average particle diameter, a granularity distribution, and the like, in addition to the indexing by means of the classification as aforementioned. In reality, however, it is rather practical and preferable industrially to prescribe the size of particles in an FeMS powder with an upper-limit value of the particles. Therefore, in the present description, the size of particles in an FeMS powder is specified by means of an upper-limit value of the particles. For example, “a particle diameter being 45 μm or less” specifies that a maximum particle diameter is 45 μm or less, which will be represented as “−45 μm” whenever appropriate.

(5) It is suitable that the raw-material powder can contain a C-system powder as another reinforcement powder, in addition to an FeMS powder. Although C in the ferrous sintered alloy can be supplied from an Fe-system powder or an FeMS powder as well, it is preferable to let a C-system powder be present mixedly in the raw-material powder separately or independently to suppress the hardening of Fe-system powders, or to make the compositional adjustment of the C amount easier. As such a C-system powder, a graphite powder (or “Gr” powder) in which C accounts for 100% virtually is suitable, although it is possible to employ Fe—C alloy powders (or cementite powders), various carbide powders, and the like.

In any event, it is suitable that the raw-material powder can be prepared eventually so that Mn makes from 0.5 to 1.5%, Si makes from 0.15 to 0.6%, and C makes from 0.2 to 0.9%, when the entire ferrous sintered alloy is taken as 100%.

<<Production Steps>>

Since the process for producing ferrous sintered alloy according to the present invention comprises a compacting step and a sintering step mainly, explanations will be made on these steps in this order.

<Compacting Step>

(1) The compacting step is a step of pressure compacting the above-described raw-material powder in which an Fe-system powder is mixed with a reinforcement powder, thereby turning the raw-material powder into a powder compact. A compacting pressure on this occasion, a density of the resulting powder compact (or a green-compact density ratio), a configuration of the resultant powder compact, and the like, do not matter at all.

However, it is allowable to set the compacting pressure and green-compact density to such an extent at least that the resulting powder compact does not collapse readily. For example, it is preferable that the compacting pressure can be 350 MPa or more, 400 MPa or more, 500 MPa or more, and further 550 MPa or more. When referring to the green-compact density ratio, it can preferably be 80% or more, 85% or more, and further 90% or more. The higher the compacting pressure and green-compact density ratio become, the more likely it is that the ferrous sintered alloy with higher strength can be obtained; however, it is permissible to select an optimum compacting pressure, or an optimum green-compact density ratio in compliance with applications and specifications of the ferrous sintered alloy. Moreover, it is allowable even if the compacting step can be performed by either cold compacting or warm compacting, and it is even permissible to add an internal lubricant agent in the raw-material powder. In the case of adding an internal lubricant agent, one which includes the internal lubricant agent is considered the raw-material powder.

(2) The present inventors, as there is a disclosure in above-mentioned Patent Literature No. 3, established a compacting method for powder compact, the compacting method which makes ultra-high-pressure compaction, which transcends general compacting pressures, feasible. In accordance with this compacting method, powder compacting is made feasible at such ultra-high pressures as 750 MPa or more, 800 MPa or more, 900 MPa or more, 1,000 MPa or more, 1,200 MPa or more, 1,500 MPa or more, and further about 2,000 MPa. The density of a powder compact, which can be obtained by means of this, can reach 96% or more, 97% or more, 98% or more, and further even up to 99%. This compacting method (hereinafter, being referred to as “die-wall lubrication warm pressure compaction method” wherever appropriate) can be summarized as follows.

The die wall lubrication warm pressure compaction method (i.e., a compacting step) comprises a filling step of filling said raw-material powder in a die with a higher fatty acid-system lubricant applied on the inner face, and a warm pressure compacting step of generating a metallic soap film on the surface of the raw-material powder, which contacts with the die inner face, by warm pressurizing the raw material powder that is disposed within this die.

In accordance with this compacting method, even when making the compacting pressure lager considerably, such drawbacks that occur in general compacting methods do not arise. Concretely speaking, the galling between the raw-material powder and the inner face of the die, the excessively enlarging ejection force, the degradation of the die longevity, and the like, can be restrained. Hereinafter, the filling step and warm pressure compacting step of this compacting method will be explained in more detail.

(a) Filling Step

Before filling the raw material powder in a die (e.g., a cavity), a higher fatty acid-system lubricant agent is applied on the inner face of a die (i.e., an applying step). It is allowable that the higher fatty acid-system lubricant employed herein, in addition to a higher fatty acid itself, can even be the metallic salts of higher fatty acids as well. As for the metallic salts of higher fatty acids, lithium salts, calcium salts, or zinc salts, and the like, are available. In particular, lithium stearate, calcium stearate, zinc stearate, and so forth, are preferable. In addition to these, it is also possible to use barium stearate, lithium palmitate, lithium oleate, calcium palmitate, calcium oleate, and so on.

The applying step, for example, can be carried out by spraying a higher fatty acid-system lubricant agent, which is dispersed in water, an aqueous solution or an alcoholic solution, and the like, into a heated die. When a higher fatty acid-system lubricant agent is dispersed in water, and so forth, it is likely to spray the higher fatty acid-system lubricant agent onto the inner face of a die uniformly. When it is sprayed into a heated die, the water content, and so on, evaporate quickly, and accordingly the higher fatty acid-system lubricant agent adheres onto the inner face of the die uniformly. Regarding the heating temperature of a die, although it is preferable to set it in view of the temperature at the warm pressure compacting step being described later, heating it in advance to 100° C. or more, will suffice, for instance. In reality, however, in order to form a uniform film of a higher fatty acid-system lubricant agent, it is preferable to set that heating temperature to less than the melting point of the higher fatty acid-system lubricant agent. For example, in a case of using lithium stearate as a higher fatty acid-system lubricant agent, it is allowable to set that heating temperature to less than 220° C.

Note that, in dispersing a higher fatty acid-system lubricant agent in water, and the like, it is preferable to include the higher fatty acid-system lubricant agent in a such proportion as from 0.1 to 5% by mass, and further from 0.5 to 2% by mass, when the entire mass of that aqueous solution is taken as 100% by mass, because a uniform lubricant film is formed on the inner face of a die.

Moreover, in dispersing a higher fatty acid-system lubricant agent in water, and the like, when a surfactant agent is added to that water in advance, it is possible to intend to disperse the higher fatty acid-system lubricant agent uniformly. As for such a surfactant agent, it is possible to use alkyl phenol-system surfactant agents, polyoxyethylene nonyl phenyl ether (EO) 6, polyoxyethylene nonyl phenol ether (EO) 10, anionic nonionic-type surfactant agents, boric acid ester-system emulbon T-80, and so forth, for instance. It is even allowable to combine two or more members of these to employ. For example, when lithium stearate is used as a higher fatty acid-system lubricant agent, it is preferable to use three members of the surfactant agents, polyoxyethylene nonyl phenyl ether (EO) 6, polyoxyethylene nonyl phenol ether (EO) 10 and boric acid ester-system emulbon T-80, simultaneously. It is because, in this case, the dispersibility of lithium stearate in water, and so on, is activated all the more, compared with the case where only one member of them is added.

In order to obtain an aqueous solution of a higher fatty acid-system lubricant agent whose viscosity fits for spraying, it is preferable to set the proportion of a surfactant agent to from 1.5 to 15% by volume when the entirety of that aqueous solution is taken as 100% by volume.

In addition to this, it is allowable to add a small amount of an antifoaming agent (a silicon-system antifoaming agent, and the like, for instance). It is because, if the bubbling of an aqueous solution is vigorous, a uniform film of a higher fatty acid-system lubricant agent is less likely to be formed on the inner face of a die when spraying it. It is advisable that an addition proportion of an antifoaming agent can be from 0.1 to 1% by volume approximately, for instance, when the entire volume of that aqueous solution is taken as 100% by volume.

It is suitable that the particles of a higher fatty acid-system lubricant agent, which is dispersed in water, and the like, can have a maximum particle diameter that is less than 30 μm. It is because, when the maximum particle diameter becomes 30 μm or more, the particles of a higher fatty acid-system lubricant agent are likely to precipitate in an aqueous solution so that it becomes difficult to apply the higher fatty acid-system lubricant agent onto the inner face of a die uniformly.

For the application of an aqueous solution, in which a higher fatty acid-system lubricant agent is dispersed, it is possible to carry it out, using spraying guns, electrostatic guns, and the like, for coating, for instance. Note that, as a result that the present inventors examined by means of experiments the relationship between the applying amount of higher fatty acid-system lubricant agents and the ejection force for powder compacts, it is preferable to have a higher fatty acid-system lubricant agent adhered on the inner face of a die so that the film thickness becomes from 0.5 to 1.5 μm approximately.

(b) Warm Pressure Compacting Step

It is believed that, when a raw-material powder, which has been filled in a die in which a higher fatty acid-system lubricant agent is applied on the inner face, is warm pressure compacted, a metallic soap film is formed on the surface of the raw-material powder (or the powder compact), which contacts with the die inner face, so that ultra-high-pressure compacting at industrial level has become feasible by means of the presence of this metallic soap film. This metallic soap film bonds firmly to the surface of that powder compact, and demonstrates far better lubricating performance than that of the higher fatty acid-system lubricant agent, which has been adhered on the inner surface of the die. As a result, it sharply reduces the frictional force between the contact face of the inner face of the die and the contact face of the outer face of the powder compact, and does not cause galling, and the like, despite high-pressure compacting. Moreover, it is possible to take the resulting powder compact out of the die with a very low ejection force, and accordingly the die longevity's excessive shortening has disappeared as well.

The metallic soap film is an iron salt film of a higher fatty acid, which is formed by a mechanochemical reaction that has taken place between a higher fatty acid-system lubricant agent and Fe in a raw-material powder under warm high pressure, for instance. A representative example of this is an iron stearate film, which has been generated when lithium stearate or zinc stearate, namely, a higher fatty acid-system lubricant agent, reacts with Fe.

It is allowable that “warm,” which is referred to in the present step, can be such an extent of heated state that the reaction between a raw-material powder and a higher fatty acid-system lubricant agent can be facilitated. Roughly speaking, it is permissible to set the compacting temperature to 100° C. or more. However, from the viewpoint of preventing the degenerative change of a higher fatty acid-system lubricant agent, it is allowable to set the compacting temperature to 200° C. or less. It is more suitable that the compacting temperature can fall within a range of from 120 from 180° C.

It is allowable that “pressurizing,” which is referred to in the present step, can be determined appropriately within a range where a metallic soap film is formed while considering the specifications of the iron-based sintered alloy. Considering the die longevity and the productivity, it is preferable to set the upper limit of that compacting pressure to 2,000 MPa. When the compacting pressure becomes 1,500 MPa approximately, the density of the obtained powder compact, too, approaches the true density (becomes from 98 to 99% by green-compact density ratio), and no further high-densification can be desired even when pressurizing it to 2,000 MPa or more.

Note that, when using this die wall lubrication warm pressure compaction method, it is not necessary to employ an internal lubricant agent, and so a powder compact with much higher density is obtainable. Moreover, when that powder compact is sintered, there is no such a case that the inside of a furnace is polluted being accompanied by the decomposition, emission, and the like, of an internal lubricant agent. However, it should be notified that, in the present invention, the employment of an internal lubricant agent shall not be excluded.

<Sintering Step>

The sintering step is a step in which a powder compact being obtained in the compacting step is sintered by heating it in an oxidation preventive atmosphere. The sintering temperature and sintering time are selected appropriately, considering the desired characteristics, productivity, and the like, of the ferrous sintered alloy. The higher the sintering temperature is in a shorter period of time the high-strength ferrous sintered alloy is obtainable. In reality, however, when the sintering temperature is too high, liquid phases arise, or dimensional contraction becomes large so that it is not preferable. When the sintering temperature is too low, the diffusion of alloying elements becomes insufficient so that it is not preferable. Moreover, the sintering time becomes longer, and accordingly the productivity of the ferrous sintered alloy declines. Consequently, it is preferable that the sintering temperature can be 900° C. or more, and further 950° C. or more; and it is preferable to be 1,400° C. or less, and further 1,350° C. or less.

In particular, when obtaining the ferrous sintered alloy with higher strength, it is allowable to set the sintering temperature to 1,000° C. or more, 1,100° C. or more, and further 1,150° C. or more. However, when using an FeMS powder with smaller granularity (concretely speaking, a fine power that has been classified to 8 μm or less, and further to 5 μm or less), the ferrous sintered alloy with higher strength is obtainable at a sintering temperature of 1,025° C. or more, and further 1,075° C. or more. When using an Fe-system powder with smaller granularity (concretely speaking, a fine power that has been classified to 70 μm or less, and further to 65 μm or less) along with that FeMS powder with smaller granularity, the ferrous sintered alloy with higher strength is obtainable at a sintering temperature of 950° C. or more, and further 1,050° C. or more.

It is allowable to set the sintering time to from 0.1 to 3 hours, and further from 0.1 to 2 hours, while considering the sintering temperature, the specifications, productivity and costs of the ferrous sintered alloy, and the like.

(2) It is allowable that the sintering atmosphere can be an oxidation preventive atmosphere. Mn and Si, the alloying elements, have extremely strong affinity to O, and so they are elements that are very likely to be oxidized. In particular, since an FeMS powder like the present invention has a lower oxide-formation free energy than those of the simple substances of Mn and Si, there is such a fear that it reacts even with scant O within a heating furnace to form the oxides of Mn and Si inside the sintered body. The intervention of such oxides is not preferable, because it deteriorates the mechanical properties of the ferrous sintered alloy. Hence, it is preferable that the sintering atmosphere can be an oxidation preventive atmosphere, such as a vacuum atmosphere, an inert gas atmosphere and a nitrogen gas atmosphere.

Moreover, when residual oxygen (or an oxygen partial pressure) in such an atmosphere matters, it is allowable to employ a reducing atmosphere in which a hydrogen gas (high-purity hydrogen gas, which is purified to a low dew point (−30° C. or less, for instance)) is mixed with a nitrogen gas in an amount of a few % by volume (from 2 to 10% by volume when the entirety is taken as 100% by volume, for instance).

In a case where the employment of a hydrogen gas is not preferable, it is advisable to carryout the sintering step according to the present invention within an inert-gas atmosphere with an ultra-low oxygen partial pressure in which the oxygen partial pressure is equivalent to 10⁻¹⁹ Pa or less (100 ppm or less by CO concentration). In such an inert gas atmosphere with an ultra-low oxygen partial pressure, even when an FeMS powder reacts with O, which has adhered on a raw-material powder, and the like, so that composite oxides, and so forth, are formed, those composite oxides are decomposed furthermore. As a result, the ferrous sintered alloy with a healthy structure is obtainable, structure in which no oxides, and so on, intervene. Note that a continuous sintering furnace, which materializes an inert-gas atmosphere with an ultra-low oxygen partial pressure (or N₂ gas), is commercially available (OXYNON furnace produced by KANTO YAKIN KOGYO Co., LTD.).

(3) In addition, it is even advisable to carry out sinter hardening for carrying out hardening by means of cooling that follows heating at the sintering step. In the sintering step, a powder compact is usually heated to such a high sintering temperature (from 1,050 to 1,350° C., and further from 1,100 to 1,300° C., for instance) that is higher than the Al transformation temperature (about 730° C. or more) (i.e., a heating step). Then, sinter hardening is done by quenching a sintered body being heated herein from the sintering temperature to the vicinity of room temperature (or down to the Ms point or less) (i.e., a cooling step). It is preferable that a cooling rate on that occasion can be from 0.5 to 3° C./second. The upper and lower limits of this rate are selectable arbitrarily within that numerical range, it is preferable especially that numerical values, which are selected arbitrarily from the group consisting of 0.5° C./second, 0.7° C./second, 2° C./second and 2.5° C./second, can make the upper and lower limits. A faster cooling rate is preferable, because the faster it is the more securely hardening is done; however, in accordance with the production process according to the present invention, hardening can be done sufficiently even when the cooling rate is slower. Consequently, in accordance with the present invention, a forcible cooling apparatus for carrying out quenching is not necessarily needed, and so it is possible to intend to lower costs in terms of facilities as well. Note that such a tendency is prominent when the ferrous sintered alloy further includes Cr and Mo in addition to C, Mn and Si.

<<Ferrous Sintered Alloy>>

(1) In the ferrous sintered alloy according to the present invention, the highness/lowness of its density does not matter. That is, like the conventional ferrous sintered alloys, it is even allowable to be a low-density ferrous sintered alloy that is made by sintering a powder compact being compacted with a general-purpose compacting pressure, or it is also permissible to be a high-density ferrous sintered alloy that is made by sintering a high-density powder compact being subjected to high-pressure compacting using the above-described die-wall lubrication warm pressure compaction method. In either of the cases, upgrading the ferrous sintered alloy's mechanical characteristics and dimensional stability can be intended by using an FeMS powder.

In particular, a green-compact density ratio, or a sintered-body density ratio that becomes 92% or more, 95% or more, 96% or more and further 97% or more is preferable, because the resulting strength becomes high strength that is equal to those of sintered bodies, which can be obtained by means of double-pressing and double-sintering (or 2P2S), to those of forge sintered bodies, and further to those of die-cast materials.

(2) Here, as being recited in above-mentioned Patent Literature No. 7, the present inventors had already found out that blistering (or blister) is likely to arise in a case where an ultra-high-density powder compact (whose green-compact density ratio is 96% or more, for instance). In particular, in a case of including C, such as a “Gr” powder, in a raw-material powder, such blistering is likely to occur. When such blistering occurs, it is natural, though, that the dimensional stability between before and after sintering collapses extremely.

Such blistering is caused by means of various gases, such as H₂O, CO and CO₂, which generate when moisture or oxides, and the like, which have adhered on the particulate surface of a raw-material powder, are reduced or decomposed during the heating of the sintering step. That is, it is believed that these gases are enclosed within sealed holes inside the resulting sintered body in which the respective constituent particles are put in a state of being adhered closely, and then the gases expand during the heating of the sintering step so that blistering occurs in the resultant sintered body. Of course, when a powder compact has such a low density as that of conventional one, the occurrence of blistering like above is less because those generated gases are emitted to the outside through spaces that are made between the particles of a raw-material powder.

In reality, however, in a case where an FeMS powder is used for a reinforcement powder like the present invention, Mn and Si (Si especially) in the FeMS powder function as an oxygen getter, respectively, and hence prevents the resulting sintered body from blistering. This is because Mn and Si exhibit stronger affinities to O than to C so that their oxide-generation free energies are lower.

Thus, when using an FeMS powder like the present invention, a ferrous sintered alloy that is good in terms of the dimensional stability comes to be obtainable even in a case where it has undergone high-density compaction.

(3) A metallic structure of the ferrous sintered alloy that is directed to the present invention does not matter. It is advisable to turn it into structures, such as martensite structures, bainite structures, pearlite structures, ferrite structures and composite structures of these, which are in compliance with required specifications for the ferrous sintered alloy, by adjusting a cooling rate after the sintering step, or by carrying out heat treatments independently of the sintering step. Depending on specifications and compositions of the ferrous sintered alloy, it is even allowable that heat treatment steps, such as annealing, normalizing, aging, refining (e.g., hardening, and tempering), carburizing and nitriding, can be further performed onto the ferrous sintered alloy.

(4) A form of the ferrous sintered alloy according to the present invention, and an intended use therefor do not matter. When naming an example of ferrous-sintered-alloy members that comprise the ferrous sintered alloy according to the present invention, in the automotive filed, the following are available: various pulleys, synchronizer hubs for transmissions, connecting rods for engines, hub sleeves, sprockets, ring gears, parking gears, pinion gears, and the like. In addition to them, the following are also available: sun gears, driving gears, driven gears, reduction gears, and so forth.

EXAMPLES

The present invention will be explained hereinafter in more detail while naming specific examples.

<<Preparation of Reinforcement Powder>>

(1) As a reinforcement powder to be blended to an Fe-system powder, the following were made ready: two kinds of FeMS powders with different compositions that are given in Table 1; and a Cu powder (“DistaloyACu” (or Fe-10% Cu) with particles diameters of from 20 to 180 μm, a product of HOEGANAES AB Corp.).

First of all, an FeMSII powder (or Fe—Mn—Si powder), one of the FeMS powders, was made by pulverizing an ingot (or solid metal casting) in air, ingot which was die cast in an Ar gas atmosphere and whose blended composition was Fe-50 Mn-30 Si (units: % by mass). Next, an FeMSIV powder (or Fe—Mn—Si—C powder), the other one of the FeMS powders, was made by pulverizing silicomanganese (e.g., JIS #3) in air, silicomanganese which was produced by NIHON DENKO Corp.

Any one of the powders was processed by pulverization for 30 minutes using a vibration milling machine that was manufactured by CHUO KAKOKI Co., Ltd. Those in such a state that they were as being processed by pulverization will be referred to as “as pulverized” in the present description, and in tables and diagrams that are attached to the present description. These pulverized powders were further sieved, thereby classifying them properly to FeMS powders that had such different granularities as particle diameters being less than 5 μm (or −5 μm), and the like. For reference, as can be understood from later-described Table 2, particle diameters “as pulverized” were less than 45 μm (or −45 μm).

(2) As can be apparent from Table 1, an Mn/Si composition was 1.5 in the FeMSII powder, whereas an Mn/Si composition became 4 in the FeMSIV powder.

Moreover, results of measuring the distributions of granularity for the “as pulverized” FeMSII powder and FeMSIV powder, which had undergone the same pulverizing treatment, are shown in Table 2. In the measurement, these distributions of granularity were measured by means of a laser diffraction scattering method using “MT3000II,” a micro-track granularity-distribution measuring apparatus that was produced by NIKKI-SO Co., Ltd. In Table 2, the numeric values that correspond to D10, D50 and D90 are maximum values of particle diameters in which 10%, 50% and 90% of the measured powdery particles are involved, respectively. For example, when taking a look at the FeMSIV powder, since the granularity was 11.5 (μm) at D90, it specifies that 90% of the entire particles had particles diameters of 11.5 μm or less. As can be apparent from comparing the D90 value of the FeMSII powder with that of the FeMSIV powder, the FeMSIV powder was considerably smaller in the entire granularity than was the FeMSII powder, though the same pulverizing treatment was performed onto them, and hence it is understood that the FeMS IV powder was better in terms of the pulverizability (or collapsibility) than was the FeMSII powder.

When the entire FeMS powder was taken as 100% by mass, an Fe amount in the FeMSII powder was about 16.5%, whereas an Fe amount in the FeMSIV powder was about 22.7%. Therefore, the proportion of Fe was greater in the FeMSIV powder than in the FeMSII powder. It seems that the FeMSIV powder was better than the FeMSII powder in the pulverizability nevertheless, because C existed in an amount as much as about 2.3% in the FeMSIV, in contrast to the FeMSII powder.

<<Manufacture of Test Specimens>>

Testing Example No. 1 Sample Nos. E493 Through E502, C1 and C2

In addition to the aforementioned reinforcement powders, a pure iron powder (e.g., a pure Fe powder/“ASC100.29” with diameters of from 20 to 180 μm, a product of HOEGANAES Corp.), one of the Fe-system powders, and a graphite (or “Gr”) powder (e.g., “JCPB” produced by NIHON KOKUEN Co., Ltd., and particle diameters being 45 μm or less), one of the C-system powders, were made ready. These powders were blended with zinc stearate (“ZnSt.”), one of the internal lubricant agents, variously, as shown in Table 3, and were then rotary mixed with a ball mill to prepare a variety of mixture powders (or raw-material powders).

Using various types of the mixture powders, the following were manufactured: test specimens (or fundamental test specimens: φ23 mm×10 mm in thickness) for measuring the densities and the dimensional changes before and after sintering; and test specimens (or tensile-test specimens) with a configuration illustrated in FIG. 1 for being subjected to a tensile test.

Concretely speaking, various types of the mixture powders were first pressure compacted at 588 MPa by use of dies for compaction, thereby obtaining powder compacts that possessed two sorts of said test-specimen configurations (i.e., a compaction step). These powder compacts were sintered in a 1,150-° C. nitrogen-gas atmosphere, respectively, using a continuous sintering furnace (“OXYNON FURNACE” produced by KANTO YAKIN KOGYOU Co., Ltd.) (i.e., a sintering step). A soaking holding time was set at 30 minutes, and an after-sintering cooling rate was set at 30° C./min. (or 0.5° C./sec.). Note that a CO concentration inside the sintering furnace was adapted into being an ultra-low-oxygen-partial-pressure atmosphere of from 50 to 100 ppm (equivalent to from 10⁻¹⁹ to 10⁻²¹ Pa by conversion into oxygen partial pressure).

Testing Example No. 2 Sample Nos. E503 Through E520

(1) Using an iron-alloy powder (e.g., a “CrL” powder/“AstaloyCrL” with particle diameters of from 20 to 180 μm, a product of HOEGANAES Corp.) whose componential composition was Fe-1.5% Cr-0.2% Mo (units: % by mass), instead of the pure iron powder in Testing Example No. 1, raw-material powders were prepared. In this instance, without using any internal lubricant agent, the respective powders were blended variously, as shown in Table 4, and were then rotary mixed with a ball mill to prepare a variety of mixture powders (or raw-material powders). Using these raw-material powders, powder compacts and sintered bodies were manufactured, powder compacts and sintered bodies which had the same configurations as those of the two sorts of the test specimens being specified in Testing Example No. 1.

(2) However, in the present testing example, the powder compacts were compacted by means of the following die-wall lubrication warm compaction method (i.e., a compaction step).

A TiN coating treatment had been performed onto the inner peripheral face in each of dies in advance, and then its surface roughness was adapted into being 0.4 Z. The respective dies had been heated to 150° C. with a band heater in advance. Then, an aqueous solution, in which lithium stearate (or “LiSt.”), one of the higher fatty acid-system lubricant agents, was dispersed, was applied uniformly onto the inner peripheral face of the heated dies with a spraying gun (i.e., a coating step). Thus, an LiSt film with about 1 μm approximately was formed on the inner peripheral face of the respective dies.

The aqueous solution used herein is one in which LiSt. was dispersed in one in which a surfactant agent and an antifoaming agent were added to water. For the surfactant agent, polyoxyethylene nonyl phenyl ethers (EO) 6 and (EO) 10, and boric acid ester emulbon T-80 were used, each of them was added in an amount of 1% by volume, respectively, with respect to the entire aqueous solution (i.e., 100% by volume). For the antifoaming agent, FS antifoam 80 was used, and was added in an amount of 0.2% by volume with respect to the entire aqueous solution (i.e., 100% by volume). For LiSt., one whose melting point was about 225° C. and average particle diameter was 20 μm was used. Its dispersion amount was set at 25 g with respect to 100 cm³ of the aforementioned aqueous solution. The aqueous solution, in which LiSt. was dispersed, was further subjected to a micro-fining treatment (e.g., with Teflon-coated steel balls (“Teflon” being a registered trademark) for 100 hours) with a ball mill-type pulverizing apparatus. The thus obtained stock solution was diluted by 20 times, and an aqueous solution with 1% final concentration was served for the aforementioned coating step.

Into the cavities of the respective dies, in which the uniform film of LiSt. was formed on the inner face, various kinds of the above-described raw-material powders were filled naturally (i.e., a filling step). The raw-material powders had been heated to 150° C., the same temperature as that of the dies, with a drier in advance.

The respective raw material powders, which were filled in the dies, were compacted with 784 MPa, thereby obtaining powder compacts (i.e., a warm pressure compacting step). It was possible to take any one of the powder compacts out of the dies with a lower ejection force, without causing any galling, and the like, on the inner face of the dies.

(3) The thus obtained powder compacts were sintered in the same manner as Testing Example No. 1. With respect to the respective sintered bodies, an annealing treatment was further performed at 200° C. for 1 hour in air.

Testing Example No. 3 Sample Nos. E521 Through E529

Using an iron-alloy powder (e.g., a “CrM” powder/“AstaloyCrM” with particle diameters of from 20 to 180 μm, a product of by HOEGANAES Corp.) whose componential composition was Fe-3% Cr-0.5% Mo (units: % by mass), instead of the iron-alloy powder (i.e., the CrL powder) in Testing Example No. 2, raw-material powders were prepared. Also in this instance, without using any internal lubricant agent, the respective powders were compacted by means of the same die-wall lubrication warm pressure compaction method as that of Testing Example No. 2 with use of raw-material powders in which the respective powders were blended variously as shown in Table 5 (i.e., a compaction step). Furthermore, the same sintering step and annealing step as those of Testing Example No. 2 were carried out. Thus, various types of powder compacts and sintered bodies being shown in Table 5 were manufactured.

<<Measurements>>

(1) Using the fundamental test specimens that were manufactured in the aforementioned respective testing examples, the following were found: densities (or G. D.) of the powder compacts; densities (or S. D.) of the sintered bodies; and dimensional changes between before and after sintering (e.g., outside-diameter changes: ΔD).

(2) Using the tensile-test specimens that were manufactured in the aforementioned respective testing examples, a tensile test was carried out, thereby finding tensile strengths, 0.2% proof strengths, and elongations. Moreover, a hardness in one of the side faces of the tensile-test specimens was measured with a load of 30 kg by means of a Vickers hardness meter.

The thus obtained results of the measurements for the respective test specimens are given in Table 3 and FIGS. 2 through 5 with regard to Testing Example No. 1; they are given in Table 4 and FIGS. 6 through 9 with regard to Testing Example No. 2; and they are given in Table 5 and FIGS. 10 through 13 with regard to Testing Example No. 3.

<<Evaluations>>

Testing Example No. 1

(1) Dimensional Change

As can be understood from Table 3 and FIG. 2, the less the FeMS powder was and moreover the smaller its granularity was, the smaller the dimensional change was. In particular, in a case where the fine FeMSIV powder whose particle diameter was 5 μm or less was used, the dimensional changes became comparable with those in cases where the conventional Cu powder or FeMSII powder were used.

(2) Hardness

As can be understood from Table 3 and FIG. 3, although the more the FeMS powder increased the greater the hardness became, there hardly was any difference that resulted from the granularity. Moreover, the hardness increased more in cases where the FeMSIV powder was used than in cases where the conventional Cu powder or FeMS II powder were used.

(3) Tensile Strength

As can be understood from Table 3 and FIG. 4, the more the FeMS powder increased and moreover the smaller its granularity was, the greater the tensile strength became. Moreover, provided that the blended amount was identical with each other, the FeMS powder made the tensile strength greater than the Cu powder did. In particular, in a case where the FeMSIV powder (e.g., −5 μm) was used, the tensile strength was upgraded by about 20% more than that in a case where the Cu powder was used.

In addition, in the samples in which the FeMS powder was used, the tensile strength became greater suddenly when the amount of the FeMS powder ranged from 1.5 to 2% by mass. On the contrary, the increase in the tensile strength slowed down when the amount of the FeMS powder was 2.5% by mass or more.

(4) Elongation

As can be understood from Table 3 and FIG. 5, the less the FeMS powder was and moreover the smaller its granularity was, the greater the elongation became. Moreover, provided that the blended amount was identical with each other, the FeMS powder made the elongation greater than the Cu powder did. In addition, in the samples in which the FeMS powder was used, the tensile strength became greater suddenly when the amount of the FeMS powder ranged from 1.5 to 2% by mass. On the contrary, the increase in the tensile strength slowed down when the amount of the FeMS powder was 2.5% by mass or more.

(5) From the results as above, it became apparent that, in a case where the FeMSIV powder, especially its fine powder (e.g., −5 μm), was used, dimensional changes, hardnesses and elongations, which were comparable with those in a case where the conventional Cu powder or the like was used, were exhibited; whereas tensile strengths increased remarkably.

Conversely speaking, even when making the blended amount of the FeMSIV powder less than the blended amount of the conventional Cu powder, it was understood that high strength is obtainable, high strength which is higher than that in a case where a Cu power is used. Besides, in such a case, it was ascertained that, while barely changing the hardnesses, the dimensional changes became much smaller and the elongations became much greater, thereby leading to very favorable results.

Since the FeMSIV powder is indeed more inexpensive in the raw-material cost than are the Cu powder and the FeMSII powder. Besides, since good characteristics are obtainable, good characteristics which are equal to or better than those of conventional ferrous sintered alloys, it is possible to remarkably reduce the production costs of the present ferrous sintered alloy.

Testing Example No. 2

(1) Dimensional Change

As can be understood from Table 4 and FIG. 6, the dimensional change depended neither on the particle diameter of the FeMS powder nor on the amount of “Gr,” and was stabilized between ±0.1 approximately. In particular, in a case where the fine FeMSIV powder (e.g., −5 μm) was used, the dimensional change was stabilized very much between ±0.05 approximately, regardless of the amounts of “Gr.”

(2) Hardness

As can be understood from Table 4 and FIG. 7, although the more the amount of the FeMS powder increased the larger the hardness became, the difference that resulted from the granularity and the amount of “Gr” was less.

(3) Tensile Strength

As can be understood from Table 4 and FIG. 8, when increasing the amount of the FeMSIV powder up to 1.5% by mass, the more the FeMS powder increased, the more the tensile strength also increased. However, the tensile strength showed a decreasing tendency when the amount of the FeMSIV powder increased to 1.5% by mass or more. Moreover, the smaller the granularity of the FeMSIV powder was, the larger the tensile strength tended to be. Moreover, even in any one of the cases, when the amount of the FeMSIV powder exceeded 1.0% by mass, the tensile strength of the ferrous sintered alloys exceeded 1,000 MPa.

(4) Elongation

As can be understood from Table 4 and FIG. 9, the less the FeMSIV powder was, the larger the elongation became. Moreover, the greater the amount of “Gr” was, the larger the elongation became. However, there hardly was any influence that resulted from the granularity of the FeMSIV powder.

(5) From the results as above, the following were understood: the ferrous sintered alloys, which used the fine powder (e.g., −5 μm) of the FeMSIV powder that is directed to the present testing example, hardly caused any dimensional change before and after sintering; they possessed a sufficient hardness; and concurrently they exhibited a very large tensile strength and elongation.

Since the ferrous sintered alloys that were thus better in terms of the respective characteristics were obtained by blending the FeMSIV powder in an amount of from 1 to 1.5% by mass approximately, it was understood that it is possible for ferrous sintered alloys being made by doing ultra-high-pressure compaction as well to remarkably reduce the production costs, in the same manner as in the case of Testing Example No. 1.

Testing Example No. 3

(1) Dimensional Change

As can be understood from Table 5 and FIG. 10, the dimensional change depended neither on the blended amount of the FeMS powder nor on the particle diameter, and was stabilized very much between ±0.1 approximately.

(2) Hardness

As can be understood from Table 5 and FIG. 11, although the more the amount of the FeMS powder increased the larger the hardness became, the hardness came to hardly increase when the amount of the FeMSIV powder exceeded 1% by mass. Moreover, there hardly was any change in the hardness that resulted from the granularity.

(3) Tensile Strength

As can be understood from Table 5 and FIG. 12, the tensile strength showed a tendency that was similar to that of the hardness. That is, although the more the amount of the FeMS powder increased the larger the tensile strength became, the tensile strength came to hardly increase when the amount of the FeMSIV powder exceeded 1% by mass.

However, being distinct from the hardness, the smaller the granularity was the greater the tensile strength became. And, even in any one of the cases, although the tensile strength exceeded 1,000 MPa, the tensile strength became ultra-high strength that exceeded 1,300 MPa especially in the cases where a fine powder of the FeMS IV powder was used.

(4) Elongation

As can be understood from Table 5 and FIG. 13, although the less the FeMS powder was the larger the elongation became, the elongation was nearly stabilized at around 1% approximately. And, the FeMSIV powder with smaller granularity resulted in obtaining larger elongations, though slightly.

(5) From the results as above, the following were understood: the ferrous sintered alloys, which used the FeMSIV powder that is directed to the present testing example, hardly caused any dimensional change before and after sintering; they possessed a sufficient hardness; and concurrently they exhibited a very large tensile strength. In particular, in the ferrous sintered alloys that used the fine powder of the FeMSIV powder (e.g., −5 μm), any one of the characteristics also turned into a good result. Therefore, in the present testing example as well, it is possible for ferrous sintered alloys being made by doing ultra-high-pressure compaction to remarkably reduce the production costs, in the same manner as in the case of Testing Example No. 2.

TABLE 1 Chemical Components of Reinforcement Powder (Fe—Mn—Si—C Powder) (% by mass) Mn/Si Mn Si C O N S P Fe Ratio FeMS II 50.2 32.8 0.06 0.42 Less Less 0.016 Balance 1.5 than than 0.001 0.003 FeMSIV 59.3 14.8 2.3 0.75 0.01 0.031 0.1 Balance 4.0

TABLE 2 Granularity Distribution in Reinforcement Powder When Underwent Pulverizing Treatment D10 D50 D90 FeMS II 1.0 3.8 18.5 FeMS IV 0.8 3.3 11.5

TABLE 3 Blended Composition of Raw-material Powder (Balance: Pure Fe Powder) Sintered Body (% by mass) Sintering in Nitrogen at 1,150° C. for 30 min. Reinforcement Rein- Lubri- Powder Compact Dimen- Powder force- cant Density Density sional 0.2% Tensile Elon- Sample Granu- ment “Gr” Agent Pressure G.D. S.D. Change Hardness Proof Stress Strength gation No. Type larity Powder Powder (ZnSt.) (MPa) (g/cm³) (g/cm³) ΔD (%) (Hv 30 kg) (MPa) (MPa) (%) E493 FeMSIV As Pul- 1.0 0.8 0.8 588 7.08 7.02 0.11 174 — 618 3.5 verized E494 ″ As Pul- 1.5 ″ ″ ″ 7.06 7.01 0.14 183 — 614 2.5 verized E495 ″ As Pul- 2.0 ″ ″ ″ 7.06 6.99 0.19 190 461 641 1.9 verized E496 ″ As Pul- 2.5 ″ ″ ″ 7.04 6.94 0.29 203 — 654 1.6 verized E497 ″ As Pul- 3.0 ″ ″ ″ 7.03 6.93 0.33 213 — 654 1.1 verized E498 ″ −5 μm 1.0 ″ ″ ″ 7.07 7.04 0.00 172 — 618 3.3 E499 ″ ″ 1.5 ″ ″ ″ 7.06 7.01 0.08 184 — 638 2.7 E500 ″ ″ 2.0 ″ ″ ″ 7.05 6.99 0.12 193 480 686 2.8 E501 ″ ″ 2.5 ″ ″ ″ 7.03 6.97 0.16 199 — 710 2.2 E502 ″ ″ 3.0 ″ ″ ″ 7.02 6.94 0.22 215 — 715 1.6 C1 FeMS II −45 μm  2.0 ″ ″ ″ 7.02 6.96 0.09 176 415 630 3.1 C2 Cu — 2.0 ″ ″ ″ 7.09 7.03 0.10 175 445 566 2.1

TABLE 4 Blended Composition of Raw-material Powder (Balance: CrL Powder*¹) Sintered Body (% by mass) Sintering in Nitrogen at 1,150° C. for 30 min. Reinforcement Rein- Lubri- Powder Compact Dimen- Powder force- cant Density Density sional 0.2% Tensile Elon- Sample Granu- ment “Gr” Agent Pressure G.D. S.D. Change Hardness Proof Stress Strength gation No. Type larity Powder Powder (ZnSt.) (MPa) (g/cm³) (g/cm³) ΔD (%) (Hv 30 kg) (MPa) (MPa) (%) E503 — — — 0.6 — 784 7.43 7.42 0.03 254 672 833 2.2 E504 — — — 0.8 — ″ 7.40 7.39 0.03 261 661 965 3.7 E505 FeMSIV As Pul- 0.5 0.6 — ″ 7.41 7.41 0.02 239 — 893 2.8 verized E506 ″ As Pul- 1.0 ″ — ″ 7.39 7.39 0.04 303 784 1039 2.1 verized E507 ″ As Pul- 1.5 ″ — ″ 7.37 7.37 0.08 341 — 1088 1.3 verized E508 ″ As Pul- 2.0 ″ — ″ 7.35 7.34 0.13 412 — 1019 0.9 verized E509 ″ As Pul- 0.5 0.8 — ″ 7.38 7.39 0.06 273 — 979 3.3 verized E510 ″ As Pul- 1.0 ″ — ″ 7.37 7.37 0.07 305 745 1035 2.8 verized E511 ″ As Pul- 1.5 ″ — ″ 7.35 7.35 0.12 320 — 1000 1.5 verized E512 ″ As Pul- 2.0 ″ — ″ 7.33 7.32 0.18 384 — 1063 1.0 verized E513 ″ −5 μm 0.5 0.6 — ″ 7.40 7.43 −0.03 274 — 974 2.6 E514 ″ ″ 1.0 ″ — ″ 7.38 7.40 −0.03 312 753 1049 2.2 E515 ″ ″ 1.5 ″ — ″ 7.36 7.38 −0.02 359 — 1125 1.3 E516 ″ ″ 2.0 ″ — ″ 7.35 7.36 0.03 413 — 1075 0.9 E517 ″ ″ 0.5 0.8 — ″ 7.38 7.40 0.00 271 — 952 2.8 E518 ″ ″ 1.0 ″ — ″ 7.36 7.38 0.00 305 798 1071 3.2 E519 ″ ″ 1.5 ″ — ″ 7.34 7.36 0.00 364 — 1201 1.9 E520 ″ ″ 2.0 ″ — ″ 7.33 7.34 0.04 401 — 1145 1.0 *¹⁾CrL Powder: Fe—1.5%Cr—0.2%Mo Powder

TABLE 5 Blended Composition of Raw-material Powder (Balance: CrM Powder*²⁾ Sintered Body (% by mass) Sintering in Nitrogen at 1,150° C. for 30 min. Reinforcement Rein- Lubri- Powder Compact Dimen- Powder force- cant Density Density sional 0.2% Tensile Elon- Sample Granu- ment “Gr” Agent Pressure G.D. S.D. Change Hardness Proof Stress Strength gation No. Type larity Powder Powder (ZnSt.) (MPa) (g/cm³) (g/cm³) ΔD (%) (Hv 30 kg) (MPa) (MPa) (%) E521 — — — 0.4 — 784 7.39 7.38 0.04 286 698 989 1.8 E522 FeMSIV As Pul- 0.5 ″ — ″ 7.37 7.37 0.03 332 — 1037 1.0 verized E523 ″ As Pul- 1.0 ″ — ″ 7.36 7.36 0.05 400 908 1216 1.0 verized E524 ″ As Pul- 1.5 ″ — ″ 7.34 7.34 0.08 413 — 1204 0.9 verized E525 ″ As Pul- 2.0 ″ — ″ 7.32 7.31 0.12 416 — 1221 0.9 verized E526 ″ −5 μm 0.5 ″ — ″ 7.37 7.40 −0.07 329 — 1114 1.3 E527 ″ ″ 1.0 ″ — ″ 7.35 7.37 −0.02 402 901 1327 1.3 E528 ″ ″ 1.5 ″ — ″ 7.35 7.37 −0.01 431 — 1342 1.1 E529 ″ ″ 2.0 ″ — ″ 7.31 7.33 0.03 427 — 1325 1.0 *²⁾CrM Powder: Fe—3%Cr—0.5%Mo Powder

<<Preparation of Reinforcement Powder>>

(1) As a reinforcement powder to be blended to an Fe-system powder, an FeMS powder (e.g., FeMSCII powder) shown in Table 6, and a Cu powder (“DistaloyACu” (or Fe-10% Cu) with particles diameters of from 20 to 180 μm, a product of HOEGANAES AB Corp.) were made ready.

The FeMSCII powder (or Fe—Mn—Si—C powder) was one which was made by pulverizing silicomanganese (e.g., JIS #1) in air, silicomanganese which was produced by NIHON DENKO Corp. Compared with the FeMSIV powder shown in Table 1, the contents of Mn, Si and O was great but the content of C was less in this FeMSC powder. Moreover, the Mn/Si composition became to be 4.

Any one of the powders was processed by pulverization for 30 minutes using a vibration milling machine that was manufactured by CHUO KAKOKI Co., Ltd. Those in such a state that they were as being processed by pulverization will be referred to as “as pulverized” or “as R” in the present description, and in tables that are attached to the present description. These pulverized powders were further sieved, thereby classifying them properly to FeMS powders that had such different granularities as particle diameters being less than 5 μm (or −5 μm), and the like. For reference, as can be understood from later-described Table 7, particle diameters “as pulverized” were less than 45 μm (or −45 μm).

(2) Results of measuring the distributions of granularity for the “as pulverized” FeMSCII powder, which had undergone the same pulverizing treatment, are shown in Table 7. Since the granularity of the FeMSCII powder was 7.9 (μm) at D90, it specifies that 90% of the entire particles had particles diameters of 7.9 μm or less. That is, the FeMSCII powder was considerably small in the granularity, and hence it is understood that the FeMSCII powder was good in terms of the pulverizability (or collapsibility). This seems to result from the following: an Fe amount was as less as about 15.2% in the FeMSCII powder; and moreover C existed in an amount as much as about 2%.

<<Manufacture of Test Specimens>>

Testing Example No. 4 Sample Nos. E599, E610, E657, E607, and E609

In addition to the aforementioned reinforcement powders (i.e., the FeMSCII powder or Cu powder), a pure iron powder (e.g., a pure Fe powder/“ASC100.29” with particle diameters of from 20 to 180 μm, a product of HOEGANAES Corp.), one of the Fe-system powders, and a graphite (or “Gr”) powder (e.g., “JCPB” produced by NIHON KOKUEN Co., Ltd., and particle diameters being 45 μm or less), one of the C-system powders, were made ready. These powders were blended variously, as shown in Table 8A and Table 8B, and were then rotary mixed with a ball mill to prepare a variety of mixture powders (or raw-material powders). No internal lubricant agent was used.

Using various types of the mixture powders, the following were manufactured: test specimens (or fundamental test specimens: φ23 mm×10 mm in thickness) for measuring the densities and the dimensional changes before and after sintering; and test specimens (or tensile-test specimens) with a configuration illustrated in FIG. 1 for being subjected to a tensile test.

Concretely speaking, various types of the mixture powders were first pressure compacted at 150° C. with a pressure of 588 MPa by means of the die-wall lubrication warm compaction method explained in the <Testing Example No. 2> section, thereby obtaining powder compacts that possessed two sorts of said test-specimen configurations (i.e., a warm pressure compaction step). These powder compacts were sintered at a predetermined temperature, which is selected from a range of from 900 to 1,150° C., in a nitrogen-gas atmosphere, respectively, using a continuous sintering furnace (“OXYNON FURNACE” produced by KANTO YAKIN KOGYOU Co., Ltd.) (i.e., a sintering step). A soaking holding time was set at 30 minutes, and an after-sintering cooling rate was set at 30° C./min. (0.5° C./sec.). Note that a CO concentration inside the sintering furnace was adapted into being an ultra-low-oxygen-partial-pressure atmosphere of from 50 to 100 ppm (equivalent to from 10⁻¹⁹ to 10⁻²¹ Pa by conversion into oxygen partial pressure).

Testing Example No. 5 Sample Nos. E628 Through E634, E640, E641, E643, and E645

Using a variety of Fe-system powders with different componential compositions, raw-material powders were prepared. In this instance, without using any internal lubricant agent, the respective powders were blended variously, as shown in Table 9, and were then rotary mixed with a ball mill to prepare a variety of mixture powders (or raw-material powders). The componential compositions (units: % by mass) of the employed Fe-system powders will be shown below one by one.

“DistaloyAE”: Fe-4 Ni-1.5% Cu-0.5% Mo (with particle diameters of from 20 to 180 μm); “DistaloyHP-1”: Fe-4 Ni-2% Cu-1.5% Mo (with particle diameters of from 20 to 180 μm); “AstaloyCrL”: Fe-1.5% Cu-0.2% Mo (with particle diameters of from 20 to 180 μm); “AstaloyCrM”: Fe-3% Cu-0.5% Mo (with particle diameters of from 20 to 180 μm); and “ASC100.29”: pure iron (with particle diameters of from 20 to 180 μm, alternatively, the same being classified to −63 μm). Any one of them was produced by HOEGANAES Corp.

Using various types of the mixture powders, the following were manufactured: test specimens (or fundamental test specimens: φ23 mm×10 mm in thickness) for measuring the densities and the dimensional changes before and after sintering; and test specimens (or tensile-test specimens) with a configuration illustrated in FIG. 1 for being subjected to a tensile test.

Concretely speaking, various types of the mixture powders were first pressure compacted by means of the die-wall lubrication warm compaction method explained in the <Testing Example No. 2> section, thereby obtaining powder compacts that possessed two sorts of said test-specimen configurations (i.e., a warm pressure compaction step). The pressure compaction was carried out at 150° C. with a pressure of 392 MPa, 588 MPa, 784 MPa or 1,176 MPa.

These powder compacts were sintered at 1,180° C., respectively (i.e., a sintering step). On this occasion, a soaking holding time was set at 45 minutes, and an after-sintering cooling rate was set at 100° C./min. Note that the interior of a sintering furnace was adapted into being a reducing atmosphere in which a hydrogen gas was mixed with a nitrogen gas (e.g., a mixing proportion: N₂-10% by volume of H₂, and a dew point: −30° C. or less).

Onto each of the sintered bodies, an annealing treatment was further performed at 200° C. for 1 hour in air (i.e., an annealing step).

Testing Example No. 6 Sample Nos. E877, E879, and E881

In addition to the aforementioned reinforcement powders (i.e., the FeMSCII powder or the Cu powder), a pure iron powder (e.g., a pure Fe powder/“ASC100.29” with particle diameters of from 20 to 180 μm, a product of HOEGANAES Corp.), one of the Fe-system powders, and a graphite (or “Gr”) powder (e.g., “JCPB” produced by NIHON KOKUEN Co., Ltd., and particle diameters being 45 μm or less), one of the C-system powders, were made ready. The FeMSCII powder was classified to −5 μm to use. These powders were blended with zinc stearate (“ZnSt.”), one of the internal lubricant agents, variously, as shown in Table 10, and were then rotary mixed with a ball mill to prepare a variety of mixture powders (or raw-material powders).

Using various types of the mixture powders, the following were manufactured: test specimens (or fundamental test specimens: φ23 mm×10 mm in thickness) for measuring the densities and the dimensional changes before and after sintering; and test specimens (or tensile-test specimens) with a configuration illustrated in FIG. 1 for being subjected to a tensile test.

Concretely speaking, various types of the mixture powders were first of all pressure compacted with predetermined compaction pressures by use of dies for compaction, thereby obtaining powder compacts that possessed two sorts of said test-specimen configurations (i.e., a compaction step). On this occasion, onto the raw-material powders in which the amount of the internal lubricant agent was 0.4% by mass, the dies for compaction was heated to 80° C. and then warm compaction was carried out; whereas, onto the raw-material powders in which the same was 0.8% by mass, room-temperature compaction was carried out. These powder compacts were sintered at 1,150° C., respectively (i.e., a sintering step). A soaking holding time was set at 15 minutes, and an after-sintering cooling rate was set at 30° C./min. (or 0.5° C./sec.) Note that the interior of a sintering furnace was adapted into being a reducing atmosphere in which a hydrogen gas was mixed with a nitrogen gas (e.g., a mixing proportion: N₂-3% by volume of H₂, and a dew point: −30° C. or less).

<<Measurements>>

Using the fundamental test specimens that were manufactured in Testing Example Nos. 4 through 6, the following were found: densities (or G. D.) of the powder compacts; densities of (or S. D.) of the sintered bodies; and dimensional changes between before and after sintering (e.g., outside-diameter changes: ΔD). Moreover, using the tensile-test specimens that were manufactured in Testing Example Nos. 4 through 6, a tensile test was carried out, thereby finding tensile strengths, and elongations. Moreover, a hardness in one of the side faces of the tensile-test specimens was measured with a load of 30 kg by means of a Vickers hardness meter.

The thus obtained results of the measurements for the respective test specimens are given in Table 8A and Table 8B (both of them will be simply referred to as “Table 8” combinedly), and FIG. 14 and FIG. 15 with regard to Testing Example No. 8; they are given in Table 9 with regard to Testing Example No. 5; and they are given in Table 10 with regard to Testing Example No. 6.

<<Evaluations>>

Testing Example No. 4

(1) Dimensional Change

As can be understood from Table 8, it was unlikely that the dimensional changes degraded greatly even when being sintered at temperatures that were lower than 1,150° C. It was understood that it is possible to inhibit dimensional changes by selecting an optimum sintering temperature in compliance with the granularity of a raw-material powder to be used.

(2) Hardness and Tensile Strength

In any one of the test specimens, the higher the sintering temperature was the hardness and tensile strength of the sintered body increased.

As can be seen from FIG. 14, using the FeMSCII powder as a reinforcement powder resulted in obtaining the sintered bodies having sufficient strengths even at lower sintering temperatures. In an Fe—Cu—C system like “E610,” no sintered body possessing a sufficient strength, namely, sintered body whose tensile strength exceeded 500 MPa, was obtainable when the sintering temperature was not the melting point of copper or more (i.e., 1,085° C. or higher, for instance). However, when using the FeMSCII powder as a reinforcement powder, high-strength sintered bodies were obtained even by the sintering at such a low temperature as 950° C., depending on the particle diameters of the raw-material powders.

Concretely speaking, Sample No. E657, which was made using the FeMSCII powder that had been classified to −45 μm, was turned into a sintered body, which exhibited a tensile strength that exceeded 500 MPa, by doing sintering at 1,050° C. or more. In Sample No. E607, which was made using the FeMSCII powder that had been classified to −5 μm, it was possible to predict that a sintered body, which exhibited a tensile strength that exceeded 500 MPa, was obtainable by doing sintering at a temperature that exceeded 1,000° C. Furthermore, Sample No. E609, which was made using the FeMSCII powder that had been classified to −5 μm along with the iron-system powder that had been classified to −63 μm, was turned into a sintered body, which exhibited a tensile strength that exceeded 500 MPa, by doing sintering at 950° C. or more.

(3) Elongation

As can be seen from FIG. 15, any one of the test specimens exhibited an elongation of 2% or more. In cases where the FeMSC II powder was used as a reinforcement powder, the elongations became the smallest at around 1,050° C., and were from 2 to 2.5% approximately in any one of the test specimens. At the higher or lower temperature than that temperature sintering was done, the more the elongation upgraded. However, the smaller the granularity of the iron-system powder and that of the FeMSCII powder were, the greater the rising proportions of the elongations tended to be.

(4) From the results as above, it became apparent that it is possible to make the sintering temperature lower by making the granularity of a predetermined reinforcement powder finer. In this instance, it was ascertained that a high-strength ferrous sintered body is obtainable by also making the granularity of the iron-system powder finer, even when the sintering is done at a lower temperature.

Moreover, it was understood that other characteristics, such as the dimensional change and the elongation, are unlikely to degrade, even when the sintering temperature is set at a lower temperature.

Testing Example No. 5

(1) Dimensional Change

It was possible to make high-density materials with densities of 7.6 g/cm³ approximately by setting the compacting pressure at 1,176 MPa.

Moreover, as can be understood from Table 9, the dimensional changes in cases where the test specimens were made using the FeMSC II powder were stabilized between ±0.2% approximately by making the compacting pressure higher, even when being such raw-material powders whose values of ΔD were likely to be made larger with lower compacting pressures.

(2) Hardness and Tensile Strength

As can be understood from Table 9, even when being test specimens that did not include any Cu, hardnesses and tensile strengths, which were comparable to those of test specimens that included Cu and Ni, were obtainable. In particular, Test Sample No. E634 exhibited a hardness and tensile strength that surpassed those of the samples that included Cu and Ni.

(3) Elongation

As can be understood from Table 9, the higher the compacting pressure became the larger the elongation became.

(4) From the results as above, it became apparent that, regarding ferrous sintered alloys that are made by compacting raw-material powders including predetermined reinforcement powders with ultra-high pressures, too, it is feasible to highly strengthen the ferrous sintered alloys by means of sinter-hardening treatments. And, it is understood that sintered bodies, which possess strengths that are comparable to those of ferrous sintered alloys that contain Cu and Ni, can be produced at lower cost.

Testing Example No. 6

In Testing Example No. 6, ferrous sintered alloys (i.e., “E877” and “E879”) according to the present invention were produced while setting up the composition of the raw-material powders, the compacting conditions, the sintering temperature, the sintering atmosphere, and the like, to more practical production conditions that aimed at making efficiency higher, making costs lower, and so forth.

In any one of the samples, the dimensional change was stabilized between ±0.2% approximately. Moreover, in a case where the samples, which were obtained by doing the compaction with a compacting pressure of 588 MPa, were compared with each other, “E877” and “E879” that did not include any Cu showed a better value in any one of the hardness, tensile strength and elongation than did “E881” that included Cu. The samples according to “E877” and “E879” were highly strengthened by further enhancing their compacting pressures.

Note that, from the respective evaluations based on Testing Example Nos. 1 through 5, it was understood that making the sintering temperature lower within a feasible range in the production conditions according to Testing Example No. 6, or further carrying out sinter hardening, leads to enabling the following to materialize: saving energies in the production steps, making the ferrous sintered alloys exhibit the high strengths still more; and the like.

In other words, it was understood that high-strength ferrous sintered alloys are obtainable even when being made under the actual production conditions.

TABLE 6 Chemical Components of Reinforcement Powder (e.g., Fe—Mn—Si—C Powder) (% by mass) Mn/Si Mn Si C O N P Fe Ratio FeMSC II 65.4 16.4 1.93 1.05 0.01 0.022 Balance 4.0

TABLE 7 Granularity Distribution in Reinforcement Powder When Underwent Pulverizing Treatment D10 D50 D90 FeMSC II 1.3 3.6 7.9

TABLE 8A Raw-material Powder (Balance: Fe-system Powder) (% by mass) Powder Compact Fe-system Reinforcement Compact- Sintered Body Powder Powder (FeMSC II) ing Hardness Tensile Elon- Sample Granu- Granu- Addition Cu “Gr” Pressure G.D. Sintering S.D. Δ D (Hv at Strength gation No. larity larity Amount Powder Powder (MPa) (g/cm³) Temp. (° C.) (g/cm³) (%) 30 kg) (MPa) (%) E599 as R — — — 0.8 588 7.28  900 7.24 0.23 116 321 3.4 E610 as R — — 2 0.8 ″ 7.32 ″ 7.28 0.19 99 312 4.2 E657 as R −45 μm  2 — 0.8 ″ 7.24 ″ 7.19 0.26 111 344 3.9 E607 as R −5 μm 2 — 0.8 ″ 7.23 ″ 7.19 0.25 111 346 4.5 E609 −63 μm −5 μm 2 — 0.8 ″ 7.21 ″ 7.16 0.25 116 419 5.6 E599 as R — — — 0.8 ″ 7.29  950 7.24 0.26 113 358 3.7 E610 as R — — 2 0.8 ″ 7.33 ″ 7.28 0.23 112 364 3.6 E657 as R −45 μm  2 — 0.8 ″ 7.23 ″ 7.18 0.32 142 420 3.8 E607 as R −5 μm 2 — 0.8 ″ 7.26 ″ 7.21 0.29 134 393 3.4 E609 −63 μm −5 μm 2 — 0.8 ″ 7.16 ″ 7.11 0.28 147 507 4.5 E599 as R — — — 0.8 ″ 7.31 1000 7.25 0.26 122 387 3.5 E610 as R — — 2 0.8 ″ 7.33 ″ 7.29 0.22 131 439 3.7 E657 as R −45 μm  2 — 0.8 ″ 7.23 ″ 7.16 0.36 165 483 2.9 E607 as R −5 μm 2 — 0.8 ″ 7.27 ″ 7.21 0.33 164 490 3.0 E609 −63 μm −5 μm 2 — 0.8 ″ 7.18 ″ 7.13 0.27 175 595 3.4 E599 as R — — — 0.8 ″ 7.32 1050 7.27 0.24 118 412 4.1 E610 as R — — 2 0.8 ″ 7.33 ″ 7.29 0.20 148 473 2.5 E657 as R −45 μm  2 — 0.8 ″ 7.22 ″ 7.15 0.38 179 549 2.2 E607 as R −5 μm 2 — 0.8 ″ 7.23 ″ 7.18 0.34 194 597 2.3 E609 −63 μm −5 μm 2 — 0.8 ″ 7.19 ″ 7.16 0.17 190 660 2.4

TABLE 8B Raw-material Powder (Balance: Fe-system Powder) (% by mass) Powder Compact Fe-system Reinforcement Compact- Sintered Body Powder Powder (FeMSC II) ing Hardness Tensile Elon- Sample Granu- Granu- Addition Cu “Gr” Pressure G.D. Sintering S.D. Δ D (Hv at Strength gation No. larity larity Amount Powder Powder (MPa) (g/cm³) Temp. (° C.) (g/cm³) (%) 30 kg) (MPa) (%) E599 as R — — — 0.8 588 7.31 1075 7.27 0.21 130 419 4.1 E610 as R — — 2 0.8 ″ 7.35 ″ 7.31 0.22 166 507 2.3 E657 as R −45 μm  2 — 0.8 ″ 7.23 ″ 7.17 0.36 183 593 2.0 E607 as R −5 μm 2 — 0.8 ″ 7.23 ″ 7.18 0.30 207 665 2.8 E609 −63 μm −5 μm 2 — 0.8 ″ 7.20 ″ 7.19 0.09 202 734 4.1 E599 as R — — — 0.8 ″ 7.30 1100 7.26 0.19 130 444 4.6 E610 as R — — 2 0.8 ″ 7.31 ″ 7.23 0.39 188 566 2.2 E657 as R −45 μm  2 — 0.8 ″ 7.22 ″ 7.16 0.35 197 629 2.2 E607 as R −5 μm 2 — 0.8 ″ 7.26 ″ 7.22 0.26 208 705 3.1 E609 −63 μm −5 μm 2 — 0.8 ″ 7.18 ″ 7.20 0.00 198 780 5.1 E599 as R — — — 0.8 ″ 7.30 1125 7.27 0.17 132 448 4.7 E610 as R — — 2 0.8 ″ 7.31 ″ 7.25 0.28 203 621 2.1 E657 as R −45 μm  2 — 0.8 ″ 7.23 ″ 7.17 0.32 202 667 2.4 E607 as R −5 μm 2 — 0.8 ″ 7.28 ″ 7.24 0.24 217 736 3.2 E609 −63 μm −5 μm 2 — 0.8 ″ 7.19 ″ 7.21 −0.05 207 800 5.3 E599 as R — — — 0.8 ″ 7.29 1150 7.25 0.16 138 466 4.7 E610 as R — — 2 0.8 ″ 7.34 ″ 7.30 0.23 203 645 2.3 E657 as R −45 μm  2 — 0.8 ″ 7.24 ″ 7.19 0.32 220 705 2.5 E607 as R −5 μm 2 — 0.8 ″ 7.27 ″ 7.24 0.23 217 785 3.5 E609 −63 μm −5 μm 2 — 0.8 ″ 7.19 ″ 7.22 −0.08 210 827 6.0

TABLE 9 Raw-material Powder (Balance: Fe-system Formed Body Powder) (% by mass) Powder Compact Hard- Elon- Reinforce- “Gr” Compacting ness Tensile ga- Sample Fe-system ment Pow- Pressure S.B. G.D. S.D. Δ D (Hv at Strength tion No. Powder Powder der (MPa) (%) (g/cm³) (g/cm³) (%) 30 kg) (MPa) (%) E628 Distaloy AE — 0.6% 392 0.10 7.00 7.03 −0.36 246 882 1.8 ″ ″ — ″ 588 0.15 7.32 7.35 −0.18 316 1108 2.9 ″ ″ — ″ 784 0.18 7.52 7.51 0.00 315 1090 3.8 ″ ″ — ″ 1176 0.23 7.66 7.59 0.24 328 1071 4.8 E629 Distaloy HP-1 — 0.6% 392 0.14 6.91 6.99 −0.51 295 1051 1.5 ″ ″ — ″ 588 0.18 7.26 7.30 −0.30 342 1277 2.7 ″ ″ — ″ 784 0.23 7.48 7.48 −0.11 405 1412 3.3 ″ ″ — ″ 1176 0.30 7.66 7.61 0.14 432 1522 4.3 E630 Astaloy CrL — 0.6% 392 0.10 6.78 6.82 −0.22 154 598 1.3 ″ ″ — ″ 588 0.13 7.21 7.23 −0.16 199 741 2.8 ″ ″ — ″ 784 0.17 7.42 7.45 −0.13 241 839 3.9 ″ ″ — ″ 1176 0.20 7.62 7.60 0.04 269 885 6.2 E631 ″ Cu Powder by 2% 0.6% 392 0.10 6.79 6.75 0.04 244 871 0.8 ″ ″ ″ ″ 588 0.13 7.27 7.21 0.16 366 1195 1.1 ″ ″ ″ ″ 784 0.16 7.45 7.37 0.23 364 1276 1.8 ″ ″ ″ ″ 1176 0.20 7.65 7.53 0.39 495 1392 1.3 E632 ″ FeMSC II Powder by 1% 0.6% 392 0.10 6.72 6.81 −0.48 294 878 1.0 ″ ″ ″ ″ 588 0.14 7.15 7.22 −0.31 338 1178 1.4 ″ ″ ″ ″ 784 0.19 7.40 7.45 −0.21 377 1222 2.3 ″ ″ ″ ″ 1176 0.22 7.56 7.59 −0.06 408 1302 3.2 E633 Astaloy CrM —  0.45% 392 0.09 6.68 6.72 −0.27 217 779 0.9 ″ ″ — ″ 588 0.13 7.15 7.18 −0.22 273 1006 1.5 ″ ″ — ″ 784 0.17 7.38 7.36 −0.08 356 1141 1.9 ″ ″ — ″ 1176 0.21 7.59 7.56 0.09 437 1380 1.4 E634 ″ FeMSC II Powder by 1%  0.45% 392 0.10 6.66 6.79 −0.66 273 1034 1.3 ″ ″ ″ ″ 588 0.14 7.09 7.18 −0.46 340 1351 1.8 ″ ″ ″ ″ 784 0.17 7.36 7.44 −0.31 443 1579 1.9 ″ ″ ″ ″ 1176 0.21 7.56 7.61 −0.12 511 1702 2.2 E640 ASC 100.29 — 0.8% 392 0.10 6.94 6.92 0.04 119 415 3.9 ″ ″ — ″ 588 0.15 7.29 7.27 0.14 140 508 5.8 ″ ″ — ″ 784 0.19 7.46 7.44 0.15 156 578 6.8 ″ ″ — ″ 1176 0.23 7.60 7.57 0.11 172 595 7.5 E641 ″ Cu Powder by 2% 0.8% 392 0.10 6.94 6.91 0.06 173 551 2.2 ″ ″ ″ ″ 588 0.16 7.31 7.29 0.16 200 721 3.0 ″ ″ ″ ″ 784 0.20 7.50 7.46 0.19 251 853 4.0 ″ ″ ″ ″ 1176 0.26 7.61 7.55 0.19 269 883 4.3 E643 ″ FeMSC II Powder by 2% 0.8% 392 0.11 6.89 6.92 −0.16 175 657 2.8 ″ ″ ″ ″ 588 0.16 7.22 7.25 −0.07 225 824 4.7 ″ ″ ″ ″ 784 0.19 7.39 7.39 −0.01 240 897 5.5 ″ ″ ″ ″ 1176 0.24 7.52 7.50 0.06 277 970 6.0 E645 ASC 100.29 (−63 μm) FeMSC II Powder by 2% 0.8% 392 0.10 6.77 6.93 −0.85 187 733 4.0 ″ ″ ″ ″ 588 0.15 7.18 7.28 −0.54 230 898 6.0 ″ ″ ″ ″ 784 0.18 7.36 7.45 −0.39 261 966 6.4 ″ ″ ″ ″ 1176 0.22 7.51 7.56 −0.21 282 1025 7.8

TABLE 10 Raw-material Powder (Balance: Fe-system Formed Body Powder) (% by mass) Powder Compact Hard- Elon- “Gr” Lubricant Compacting ness Tensile ga- Sample Reinforcement Pow- Agent Pressure Compacting S.B. G.D. S.D. Δ D (Hv at Strength tion No. Powder der (ZnSt.) (MPa) Temperature (%) (g/cm³) (g/cm³) (%) 30 kg) (MPa) (%) E877 FeMSC II Powder by 2% 0.8 0.8 490 Room Temperature 0.23 6.95 6.88 0.20 187 638 1.7 ″ ″ ″ ″ 588 ″ 0.27 7.04 6.98 0.19 197 669 2.1 ″ ″ ″ ″ 686 ″ 0.30 7.10 7.05 0.18 204 695 2.0 E879 ″ ″ 0.4 392 80° C. 0.15 6.88 6.82 0.25 184 608 1.9 ″ ″ ″ ″ 588 ″ 0.22 7.18 7.12 0.25 217 753 2.3 ″ ″ ″ ″ 784 ″ 0.27 7.30 7.26 0.23 241 805 2.2 E881 Cu Powder by 2% 0.8 0.8 588 Room Temperature 0.24 7.12 7.04 0.20 185 589 1.8 

1. A process for producing ferrous sintered alloy, the process being characterized in that it is a process for producing ferrous sintered alloy being equipped with: a compaction step of pressure compacting a raw-material powder in which an Fe-system powder comprising at least either one of pure iron or an iron (Fe) alloy is mixed with a reinforcement powder containing an alloying element other than Fe, thereby turning the raw-material powder into a powder compact; and a sintering step of heating the powder compact in an oxidation preventive atmosphere, thereby sintering the powder compact; and said reinforcement powder is an Fe—Mn—Si—C powder comprising an Fe alloy or an Fe compound that includes: manganese (Mn) in an amount of from 58 to 70% by mass (hereinafter being simply referred to as “%”); silicon (Si) in an amount making a compositional ratio of the Mn with respect to the Si (i.e., Mn/Si) that is from 3.3 to 4.6; and carbon (C) in an amount of from 1.5 to 3%; when the entirety is taken as 100%.
 2. The process for producing ferrous sintered alloy as set forth in claim 1, wherein a particle diameter of said Fe—Mn—Si—C powder is 45 μm or less.
 3. The process for producing ferrous sintered alloy as set forth in claim 1, wherein a blended amount of the Fe—Mn—Si—C powder in said raw-material powder is from 0.05 to 3% when the entirety of said raw-material powder is taken as 100%.
 4. The process for producing ferrous sintered alloy as set forth in claim 1, wherein said raw-material powder further includes a graphite (or “Gr”) powder.
 5. The process for producing ferrous sintered alloy as set forth claim 1, wherein said Fe—Mn—Si—C powder has oxygen (O) in an amount of 1.5% or less when the entirety is taken as 100% by mass.
 6. The process for producing ferrous sintered alloy as set forth in claim wherein said raw-material powder is prepared in order that: Mn makes from 0.1 to 2.1%; Si makes from 0.05 to 0.6%; and C makes from 0.1 to 0.9%; when the entirety of said ferrous sintered alloy is taken as 100%.
 7. The process for producing ferrous sintered alloy as set forth in claim 1, wherein the oxidation preventive atmosphere of said sintering step is a reducing atmosphere in which a hydrogen gas is mixed in a nitrogen gas in an amount of from 2 to 10% by volume when the entirety is taken as 100% by volume.
 8. The process for producing ferrous sintered alloy as set forth in claim 1, wherein the oxidation preventive atmosphere is an inert gas atmosphere with such an extremely-low oxygen partial pressure that the oxygen partial pressure corresponds to 10⁻¹⁹ Pa or less.
 9. The process for producing ferrous sintered alloy as set forth in claim 1, wherein the sintering step comprises: a heating step of heating said powder compact to a temperature of from 900 to 1,400° C., thereby turning the powder compact into a sintered body; and a cooling step of cooling the sintered body as heated at a cooling rate of from 0.1 to 3° C./second.
 10. The process for producing ferrous sintered alloy as set forth in claim 1, wherein said raw-material powder includes chromium (Cr) in an amount of from 0.3 to 5% and/or molybdenum (Mo) in an amount of from 0.1 to 2% when the entirety is taken as 100%.
 11. The process for producing ferrous sintered alloy as set forth in claim 10, wherein said Cr and/or Mo is included in said Fe-system powder. 12.-15. (canceled) 